Foundation with pedestal and ribs for towers

ABSTRACT

A wind turbine foundation comprising a concrete support slab having a horizontal rebar grid therein, a concrete pedestal integral with the support slab and having vertical post tensioning elements therein and a plurality of concrete ribs on top of and integral with the support slab and integral with the pedestal, the ribs having rebar therein and extend outwardly from the pedestal, the pedestal, slab and ribs are connected to each other to form a monolithic foundation. The foundation design reduces the weight and volume of materials used, reduces cost, and improves heat dissipation conditions during construction by having a small ratio of concrete mass to surface area thus eliminating the risk of thermal cracking due to heat of hydration.

CROSS-REFERENCED TO RELATED APPLICATIONS

This application is a continuation of Ser. No. 15/916,724, filed Mar. 9,2018, which is a continuation of Ser. No. 15/530,081 filed Dec. 1, 2016,which is a continuation of application Ser. No. 15/137,157 filed Apr.25, 2016 now U.S. Pat. No. 9,534,405 issued Jan. 3, 2017, which is acontinuation of application Ser. No. 14/748,241 filed Jun. 24, 2015 nowU.S. Pat. No. 9,347,197 issued May 24, 2016, which is a continuation ofapplication Ser. No. 14/176,160 filed Feb. 10, 2014 now U.S. Pat. No.9,096,985 issued Jul. 4, 2015, which is a continuation of applicationSer. No. 13/319,083 filed Jul. 5, 2010 now U.S. Pat. No. 8,661,752issued Mar. 4, 2014, which claims the benefit of provisionalapplications 61/215,430 filed May 5, 2009, 61/269,800 filed Jun. 29,2009, 61/284,901 filed Dec. 28, 2009, 61/339,550 filed Mar. 5, 2010 andclaims priority from PCT/US/2010/041006 and is a continuation-in-part ofpatent application Ser. No. 12/774,727 filed May 6, 2010, which is acontinuation-in-part of patent application Ser. No. 11/859,588 filedSep. 21, 2007 which claims the benefit of provisional applications60/826,452 filed Sep. 21, 2006 and 60/954,502 filed Aug. 7, 2007.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable

BACKGROUND OF THE INVENTION I. Field of the Invention

This invention relates to building fatigue resistant foundations forsupporting columns, towers and structures under heavy cyclical loadssuch as for onshore and offshore wind turbine towers. Wind turbinesupport structures are subjected to high cyclical loading with thenumber of load cycles up to 10⁹. Therefore, the fatigue design becomesmore important for concrete construction and the influence ofmulti-stage and multi-axial fatigue loading have to be considered.Studies have recommended modification of the design rules for concreteconstruction of wind turbine foundations in order to consider theinfluence of multi-axial loading.

Wind turbine manufacturers have successfully developed large windturbines with rated power ranging from 1.5 to 10 MW, for onshore andoffshore installations. The E-126 model turbine by Enercon with a 7 MWrated power required a 29 meter diameter circular foundation with 1,400cubic meters of concrete and 120 tons of rebar. The RePower 5M turbinewith a 5 MW rated power required a 23 meter diameter circular foundationwith 1,300 cubic meters of concrete and 180 tons of rebar. The task ofbuilding such large foundations is monumental and requires a great dealof construction planning and logistics. The proposed foundation designsand their associated construction methods provide cost-effectivesolutions for such challenging foundation projects.

Some wind turbine installations that have been constructed in the last10 years in the US and Europe have encountered structural problemsstemming from thermal cracking during construction, or from fatiguecracking requiring repairs. The present invention improves the geometryof the foundation in order to enhance dissipation conditions for thetypical temperature rise due to heat of hydration after casting and alsoprovides a cost effective fatigue resistant design.

DESCRIPTION OF THE RELATED ART

Conventional gravity style foundations for large wind turbines usuallycomprise a large, thick, horizontal, heavily reinforced cast in situconcrete base; and a vertical cast in situ cylindrical pedestalinstalled over the base. There are several problems typicallyencountered during the construction of such foundations.

Fatigue resistance of such conventional footings is achieved by oversizing the structural concrete elements and the reinforcing elementssuch that the resulting stress amplitudes are small enough for thestructural elements to pass fatigue design checks.

The main problem is the monumental task of managing large continuousconcrete pours, which require sophisticated planning and coordination inorder to pour large amounts of concrete per footing, in one continuouspour, without having any cold joints within the pour.

Another problem is logistics coordinating with multiple local batchplants the delivery plan of the large number of concrete trucks to thejob site in a timely and organized manner.

A further problem is the complexity of installing the rebar assemblyinto the foundation which requires assembling two layers of steelreinforcing meshes that are two to six feet apart across the full areaof the foundation, while maintaining a strict geometric layout andspecific spacing. This rebar assembly is made of extremely long andheavy rebar which requires the use of a crane in addition to multipleworkers to install all the components of the assembly. The rebar oftenexceeds forty feet in length, thus requiring special oversized shipmentswhich are very expensive and usually require special permits. Theinstallation of the rebar is a labor intensive and time consuming taskrequiring a large number of well trained rebar placing workers.

Another important problem is the fact that the majority of theconstruction process consists of field work which can easily becompromised by weather conditions and other site conditions.

Another problem is thermal cracking of concrete due to overheating ofthe concrete mass. When concrete is cast in massive sections, thetemperature can reach high levels and the risk of thermal crackingbecomes very high. Thermal cracking often compromises the structuralintegrity of foundations as reported in many projects in Europe andNorth America.

Multi-cell caissons used in offshore installations always lackmulti-axial post-tensioning elements and their fatigue resistance reliescompletely on heavily reinforced oversized concrete elements whichinvolves expensive and labor intensive construction.

BRIEF SUMMARY OF THE INVENTION

It is desired to have a cost-effective foundation which reduces theamount of construction material used in the construction of windturbines. This can be accomplished by the use of concrete ribstiffeners, with a cast in place slab on grade element and a centralpedestal to build an integral foundation that will behave structurallyas a monolithic foundation structure. Other concrete components can beincluded such as secondary and perimeter beams, diaphragms, orintermediate stiffeners and rib stiffened or flat slab sections. Thefoundation system may use prefabricated components including rebarmeshes and cages, a pedestal cage assembly, precut post-tensioningstrands, preassembled strand bundles, precut post-tensioning ductsections and prefabricated concrete forms.

The present invention pertains to a fatigue resistant foundation forwind towers which comprises a plurality of components, namely a centralvertical pedestal, a substantially horizontal continuous bottom supportslab with a stiffened perimeter, a plurality of radial reinforcing ribsextending radially outwardly from the pedestal and a three-dimensionalnetwork of vertical, horizontal, diagonal, radial and circumferentialpost-tensioning elements embedded in the footing that keeps all thestructural elements under heavy multi-axial post compression, whichreduces stress amplitudes and deflections and allows the foundation tohave a desirable combination of high stiffness and superior fatigueresistance while improving heat dissipation conditions duringconstruction by having a small ratio of concrete mass to surface areathus eliminating the risk of thermal cracking due to heat of hydration.

Although the application is written with a wind turbine tower as thecolumn being supported by the foundation, any tower or column can beused on the foundation including but not limited to, antennas, chimneys,stacks, distillation columns, water towers, electric power lines,bridges, buildings, or any other structure using a column.

In one embodiment of the invention a wind turbine foundation has aplurality of components, namely a central vertical pedestal, asubstantially horizontal bottom support slab, and a plurality of radialreinforcing ribs extending radially outwardly from the pedestal, theribs are prefabricated and transported to job site, but the pedestal andsupport slab are poured in situ at the site out of concrete. Theprefabricated ribs 16 are equipped with load transfer mechanisms, forshear force and bending moment, along the conjunctions with the cast insitu support slab, pedestal and perimeter beams. The prefabricated ribsare also equipped at their inner ends with load transfer mechanisms, forshear force and bending moment, along the conjunctions with the cast insitu pedestal. The ribs are arranged in a circumferentially spacedmanner around the outer diameter of the pedestal cage assembly before orafter slab reinforcing steel is installed. Forms are then arranged forthe pedestal and support slab. The support slab is cast in situ bypouring concrete into the forms and then pedestal concrete is pouredinto the pedestal form over the slab. When the concrete cures thesupport slab 20 is united to the prefabricated ribs 16 and the ribs 16are also united to the pedestal 10. The final result is a continuousmonolithic polygon or circular shaped foundation wherein loads arecarried across the structure vertically and laterally through acontinuous structure by the doweled and spliced reinforcing steel barswhich are integrally cast into the pedestal 10, ribs 16 and support slab20. The combination of the high stiffness of the ribs 16, solid pedestal10 and continuous slab 20 construction across the pedestal 10, andthrough or under ribs 16, allows the slab 20 to behave structurally as acontinuous slab over multiple rigid supports resulting in small bendingand shear stresses in the slab 20, reducing deflections and increasingthe stiffness of the foundation 100, improving fatigue conditions aswell as allowing for the benefits of an economical design. Support slabreinforcing steel covers the entire footprint of the foundation andextends, without interruption, across the slab area and into thepedestal 10 to improve the structural performance of the foundation 100under different loading conditions. Perimeter beams 190 or thickenedslab edges 21 around the perimeter add stiffness and strength to thefoundation 100 and provide the benefits of a two-way slab system.Circumferential post-tensioning of the slab edge 21 is used to increasethe structural capacity of the ribs 16 and the pedestal 10 by creatingeccentric post-compression force in the ribs 16 and by reducing stressamplitudes in the slab 20, ribs 16 and pedestal 10.

The foundation of the present invention substantially reduces the amountof concrete used in a wind turbine foundations of spread footing style,simplifies the placement of rebar and concrete in the foundation, allowsfor labor and time savings and shortens the foundation constructionschedule when compared to conventional foundation designs.

This invention provides the wind energy industry with a foundationsystem suitable for utility scale wind turbines including 1.5 MW through10 MW or larger turbines, wherein the amount of cast in situ concretework is limited, and the number of concrete trucks required for thefoundation is kept to a smaller and more manageable level, and theamount of rebar used in the foundation is around 60% less thanconventional footings.

The present invention uses prefabricated components that meet size andweight limit's for standard ground freight shipping on typical roads andhighways, without resorting to special permitting for oversize oroverweight shipments, keeping in mind that the foundation width forlarge turbines can easily exceed sixty feet.

One embodiment of the invention uses specific combinations of precastcomponents with cast in situ components designed to speed upconstruction without compromising the rigidity and structural continuityand optimization of the foundation. The combination of high strength,high stiffness prefabricated ribs, solid pedestal construction andcontinuous slab construction across the pedestal, and through or underthe ribs, allows the slab to behave structurally as a continuous slabunder multiple rigid supports resulting in small bending and shearstresses in the slab, reducing deflections and increasing the stiffnessof the foundation, substantially reducing fatigue as well as allowingfor the benefits of rapid construction and economical design.

The present invention improves the geometry of the foundation in orderto enhance dissipation conditions for the heat of hydration due to thetypical temperature rise after casting. This design feature is achievedby reducing the thickness of the support slab and the ratio of concretemass to surface area, thus reducing the risk of thermal cracking andprotecting the structural integrity of the foundations.

The present invention optimizes the design support slab by configuringslab reinforcing to span between supporting ribs, and allowing it tocontinue under or across the ribs. Each slab panel may be triangular orpie-shaped and is prestressed along all three sides such that amulti-axial prestress is generated in each slab panel. Slab panels withradial and perimeter post tensioning elements form a robust horizontaltrussed diaphragm and as a result, the required slab thickness isoptimized and the amount of cast in situ concrete is reduced.

The present invention reduces the maximum rebar length for fieldinstallation to approximately half the conventional length, to roughly7.6 meters (twenty five feet), which is significantly shorter whencompared to conventional footing that may requires 15.2 to 18.3 meters(fifty to sixty foot) long reinforcing bars.

The present invention allows rib dowels, or post tensioning tendons,extending inwardly into the pedestal at one end, to continue withoutinterruption between distal ends of the foundation. As a result eachpair of ribs 16 on opposite ends of the pedestal 10 will behavestructurally as one continuous beam across the width of the foundation100.

The present invention reduces fatigue for concrete and rebar in thefoundation by minimizing stress concentrations through appropriatelyconfigured connections and component geometry. The solid and deepconstruction of the pedestal allows for great reduction of stressesacross the pedestal and at the conjunctions between the pedestal and thesurrounding slab and ribs. Dowels from the ribs into the pedestal arerelatively deep to reduce stresses in the surface zone of the pedestaland can be paired with corresponding dowels extending from the oppositeend of the foundation. The solid pedestal offers desirable bearingconditions for the tower base plate and improves the geometry as neededto minimize fatigue.

The present invention employs prestressing and/or post tensioningtechniques in order to maximize the performance of the foundation, andto extend its life span. Besides the vertical tensioning of anchorbolts, tensioning of horizontal and diagonal tendons are employed alongthe length of the concrete ribs and across the pedestal. Further,perimeter and radial post tensioning elements embedded in the slab areemployed. Post-tensioning of the ribs is designed in an eccentric mannerto counter balance and reduce the stresses from the dead loads on thefoundation. This can be accomplished by setting an eccentric posttensioning load pattern in the ribs with higher axial force at thebottom than at the top of the rib. The circumferential post tensioningload in the slab provides additional desirable eccentric prestressing ofthe ribs and the pedestal and helps increase rib load capacity and ribfatigue resistance.

OBJECTS OF THE INVENTION

An object of this invention is to provide the wind energy industry witha short construction time, reliable, and cost effective foundationsystem suitable for most wind energy projects, including projects usingthe largest utility scale turbines and tallest towers, while providing afoundation lifespan that is longer than conventional foundation systems.

Another object of this invention is to reduce the cost of wind energyprojects by realizing savings in the areas of reducing concrete andrebar quantity, reducing concrete trucking service, decreasing concretepouring and finishing, improving logistics, and reducing man-hours andcrane operations.

It is the object of this invention to provide a foundation suitable forlarge wind turbines including utility scale turbines ranging from 1.5 MWto 10 MW and larger, wherein the amount of cast in situ concrete work islimited and the number of concrete trucks and the amount of rebarrequired for the foundation is reduced to a manageable level whencompared to conventional gravity style foundations.

Another object of this invention is to improve dissipation conditionsfor the heat of hydration and the typical temperature rise aftercasting. This goal is achieved by reducing the ratio of concrete mass tosurface area. When concrete is cast in massive sections for wind towerfoundations, temperatures can reach high levels and the risk of thermalcracking becomes very high unless cooling techniques or specialadmixtures are applied. Thermal cracking often compromises thestructural integrity of the foundations.

A further object of one embodiment of this invention is to improvefoundation structural properties due to fabrication of some structuralcomponents in a fully controlled environment of a precast concrete plantor a suitable facility at or near the project site and to utilize andbenefit from advancement in concrete construction in areas such asconcrete admixtures, special cements and fiber reinforcement.

Still another object of this invention is to utilize desirable featuresand benefits associated with mass production of precast concrete such ashigh reliability and uniform consistency arid high compressive strength.

Another important object of this invention is to minimize chances forerrors in bar placement, spacing and layout by providing pre-markedspacing for splicing slab rebar with existing dowels extending fromribs.

A further object of this invention is to use light weight, smalldiameter, short and easy to handle rebar for the cast in situ concrete.

A further object of this invention is to provide the wind energyindustry with a solution for all weather foundation construction.

Still another object of this invention is to improve safety andaccessibility around foundations under construction, and reducehazardous conditions for construction crews.

A further object of this invention is to increase productivity andincrease the number of footings that can be built in a given time frameusing the same number of workers, when compared to conventionalfoundation designs built under similar conditions.

Another object of this invention is to employ prestressing and/or posttensioning techniques in order to maximize the performance of thefoundation, improve its fatigue resistance and extend its life span.

Another object of this invention is to provide the wind energy industrywith reliable and readily available designs, and optionally,prefabricated components, for every wind energy project, whereinfoundation designs are pre-approved by and coordinated with turbinemanufactures and certification agencies.

A further object of this invention is to use standard designs to reduceengineering work and simplify the permitting process, as well as improvethe project construction schedule.

Still another object of this invention is to speedup construction byusing many prefabricated components including rebar meshes and cages,bolt cage assemblies, pre-cut post-tensioning strands, preassembledpost-tensioning bundles, pre-cut post-tensioning duct sections andprefabricated concrete forms and optionally, precast ribs.

It is also the object of this invention to provide wind energydevelopers with the ability to select pre-approved complete foundationdesigns for wind turbine foundations based on project and site variablesincluding turbine model and tower height; site geotechnicalcharacteristics; and desired foundation styles such as gravity, anchoredor piling support foundations.

Another object of this invention is to provide foundation contractorswith the convenience and economy of using commercially availableprefabricated components with complete assembly and detail drawings thatcan be delivered to any project site with short lead times.

A further object of this invention is to improve the quality andproductivity of foundation construction due to experience gained frompracticing standard construction techniques with repetitive productionsteps.

Still another object of his invention is to produce foundation designssuitable for shallow and deep offshore installations.

Another object of this invention is to use the modular foundation systemfor other tower structures such as chimneys, stacks, distillationcolumns, telecommunication towers, and water towers.

Yet another object of the invention is to improve tower base bearingresistance in concrete pedestals supporting wind towers such that itbecomes possible to build the pedestal and the foundation with concretehaving the same compressive strength without increasing the diameter ofthe pedestal.

Another object of the invention is to build wind tower foundations inone continuous concrete pour.

Another object of the invention is to independently produceprefabricated components for offshore foundations to be assembled on abarge without having the critical path of completing a first componentbefore a second component can be constructed.

Other objects, advantages and novel features of the present inventionwill become apparent from the following description of the preferredembodiments when considered in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of the foundation.

FIG. 2 is a sectional elevation view of the foundation cut near theribs.

FIG. 3 is sectional elevation view of the foundation.

FIG. 4 is a section detail of a cast-in-situ slab and pedestal showingthe different reinforcing groups in both elements

FIG. 5 is a partial sectional elevation of the foundation showingdifferent bar reinforcing groups of a rib along with dowels forconnecting to a pedestal and a slab.

FIG. 6 is a partial plan view of the foundation showing different barreinforcing groups of a rib along with dowels for connecting to apedestal and a slab.

FIG. 7 is a partial sectional elevation of the foundation showingdifferent post-tension reinforcing groups of a rib along withpost-tension dowels for connecting to a pedestal and a slab.

FIG. 8 is a partial plan view of the foundation showing differentpost-tension reinforcing groups of a rib along with dowels forconnecting to a pedestal and a slab.

FIG. 9 is a slab reinforcing plan view showing the different reinforcinggroups of a lower slab reinforcing assembly along with perimeterreinforcing cages.

FIG. 10 is a slab reinforcing plan view showing the differentreinforcing groups of an upper slab reinforcing assembly along withperimeter reinforcing cages.

FIG. 11 is a vertical section view of a pedestal showing the differentbar reinforcing groups of a pedestal including the two confinement cagessurrounding the anchor bolt assembly. Rib and slab dowels are not shownfor clarity.

FIG. 12 is a partial vertical section view of the foundation showingdifferent bar reinforcing groups of a pedestal including the twoconfinement cages surrounding the anchor bolt assembly. Rib and slabdowels are not shown for clarity.

FIG. 13 is a plan view of a foundation with a heavily post tensionedring beam using ring anchors and 180-degree ring tendons.

FIG. 14 is a plan view of a partially prefabricated foundation with aheavily post tensioned ring beam extending above the slab and using ringanchors and 180-degree ring tendons with anchor blisters extending fromthe foundation. General arrangement of temporary rib support and drainsis shown.

FIG. 15 is a sectional elevation view of a partially prefabricatedfoundation with a heavily post tensioned ring beam extending above theslab and using ring anchors and 180-degree ring tendons with anchorblisters extending from the foundation. General arrangement of temporaryrib support and their corresponding sub-footings is shown.

FIG. 16 is a plan view of a partially prefabricated foundation with aheavily post tensioned ring beam extending above the slab and using ringanchors and 180-degree ring tendons with anchor blisters extending fromthe foundation. General arrangement of temporary rib support and drainsis shown.

FIG. 17 is a partial plan view of a partially prefabricated foundationshowing a general arrangement of lower radial post tensioning ducts inthe rib inner zones and across the pedestal.

FIG. 18 is a partial plan view of a partially prefabricated foundationshowing a general arrangement of upper radial post tensioning ducts inthe rib inner zones and across the pedestal.

FIG. 19 is a partial plan view showing a general arrangement of radialpost tensioning duct spacing in the pedestal.

FIG. 20 is a partial section view of a partially prefabricatedfoundation showing a general arrangement of upper and lower radial posttensioning ducts in the rib inner zones and across the pedestal.

FIG. 21 is an elevation view of a prefabricated rib showingpost-tensioning ducts and an anchor arrangement. Rib bottom dowels forslab connection and concrete shear key corrugations are shown.

FIG. 22 is a plan view of a prefabricated rib showing post-tensioningducts and an anchor arrangement. Rib side dowels for the ring beamconnection and concrete shear key corrugations are shown.

FIG. 23 is a plan view of a foundation having a hexagonal footprint anda thickened and heavily post tensioned slab edge. A simplified forcediagram shows the cumulative resultant (R) of radial post tension (PT1)and perimeter post tension (PT2). The resultant is the effectivepost-tension force acting at rib end is defined by the equation: R=PT1+2PT2 (cos a), where (a) is the angle between PT1 and PT2. In thisconfiguration all ribs are subjected to equal heavy eccentric postcompression stresses that maximize rib structural resistant to governingtower loads.

FIG. 24 is the effective rib cross section showing the neutral axis 16 nand the eccentricity (eR) of the effective post tensioning force R.

FIG. 25a is a diagram that shows a foundation cross section posttensioning, before a tower dead load and backfilling are added.

FIG. 25b is a diagram that shows cambers in the foundation of FIG. 25 a.

FIG. 25c is a diagram that shows a foundation cross section posttensioning, after a tower dead load and backfilling are added.

FIG. 25d is a diagram that shows cambers in the foundation of FIG. 25c ,after tower dead load and backfilling are added.

FIG. 26 is a section view of a foundation having an embedded towersection in the pedestal.

FIG. 27 is a connection detail showing an alternative doweling methodbetween a prefabricated rib and a slab where dowels extend down from therib into grouted sleeves arranged in a slab.

FIG. 28 is a connection detail showing an alternative doweling methodbetween a cast-in-situ rib and a slab where dowels extend up from theslab into rib forms to mesh with rib reinforcing elements.

FIG. 29 is a connection detail showing an un-bonded rock anchorconnection to the foundation with bearing and tensioning elementsreceiving an anchor bolt extending through vertical holes in thefoundation.

FIG. 30 is a connection detail showing a bonded rock anchor connectionto the foundation with bearing and tensioning elements receiving ananchor bolt extending through vertical holes in the foundation. Theanchor is tensioned and grouted to a specific depth. The anchor may beconfigured to function as a pile anchor.

FIG. 31 is a section of a foundation comprising a concrete sternextending above the pedestal and the post tension duct with loop anchorsare a arranged to facilitate the vertical post tensioning of the sternand the pedestal.

FIG. 32 is a detail that shows perimeter and radial post tensioning in afoundation with a cantilevered slab edge that extends beyond a thickenedslab ring.

FIG. 33 is a detail that shows perimeter and radial post tensioning in afoundation with a thickened slab edge.

FIG. 34 is a detail that shows a side view of a prefabricated rib to aprefabricated perimeter beam connection.

FIG. 35 is a detail that shows a top view of a prefabricated rib to aprefabricated perimeter beam connection.

FIG. 36 is detail that shows a rib being temporary supported by a set ofrib supports with through bolts extending through holes in the ribs andremovable and reusable assembly that connects to a lower supports onsub-footings. Cotter pins are used to secure the top assembly to thebottom support.

FIG. 37 is a plan view of an anchor bolt template fitted with bolt holesmatching that of the tower base flange and having means for holding atleast three leveling bolts with inserts.

FIG. 38 is a detail of the leveling bolts and corresponding insertsduring a concrete pour.

FIG. 39 is a detail of the leveling bolts and corresponding insertsduring leveling and grouting of a tower base flange.

FIG. 40 is a perspective view of the bottom segment of a partiallyprefabricated offshore foundation ready to receive a prefabricated metalor concrete stem atop the pedestal.

FIG. 41 is a perspective view of a completed partially prefabricatedoffshore foundation with a prefabricated concrete stem atop thepedestal. Vertical post tensioning elements with marine groutingmethods, such as grouted loop anchors are used to connect theprefabricated stem to the pedestal.

FIG. 42 is an elevation view of art offshore foundation duringinstallation. The foundation is stabilized with ballast over the baseand inside the stem. Scour protection measures are added around theperimeter of the base.

FIG. 43 is an elevation view of a foundation with a prefabricatedsegmented concrete stem. The foundation is supported by micro-piles oranchors.

FIG. 44 is a perspective view of the foundation comprising a prestressedconcrete base and a lattice steel tower with a wind tower receivingadaptor at the top.

FIG. 45 is a perspective view of the foundation.

FIG. 46 is a perspective view of the foundation.

FIG. 47 is a connection detail of an L-shaped, prefabricated perimeterbeam to a cast-in-situ slab.

FIG. 48 is a perspective view of the prefabricated rib foundation optionshowing the rebar before pouring the concrete.

FIG. 49 is a perspective view of a foundation in with a concrete stemextending above the foundation. The vertically prestressed stem is madewith prefabricated concrete segments or cast in place concrete. Thisconfiguration is suitable for offshore wind towers or hybrid concretesteel wind towers. The pedestal has a solid core and the stem has ahollow core that can be filled with ballast at the offshore installationsite.

FIG. 50 is a perspective view of a foundation.

FIG. 51 is a perspective view of the foundation during construction withslab concrete in place and the pedestal ready for a concrete pour

FIG. 52 is a perspective view of the bolt assembly and alignmentapparatus.

FIG. 52a is the rod support for the bolt assembly and alignmentapparatus.

FIG. 53 is a plan view of the foundation showing different groups ofreinforcing and post-tensioning elements in the slab.

FIG. 54 is a perspective view of a prefabricated rib and forms forforming the pedestal and slab.

FIG. 55 is an inner perspective view of a prefabricated rib showing ribdowels and connections to the pedestal and the slab.

FIG. 56 is a perspective view of the pedestal cage assembly with anchorbolts and reinforcing cages around the anchor bolt assembly.

FIG. 57 is a perspective view of a completed foundation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention pertains to a wind turbine foundation. Thefoundation comprises a plurality of components, namely a centralvertical pedestal, a substantially horizontal bottom support slab, and aplurality of radial reinforcing ribs extending radially outwardly fromthe pedestal. The ribs may be prefabricated and transported to job site,but the pedestal and support slab are poured in situ at the site out ofconcrete. Alternatively the ribs may be cast in situ.

The present invention pertains to a fatigue resistant foundation 100 forwind towers which comprises a plurality of components, namely a centralvertical pedestal 10, a substantially horizontal continuous bottomsupport slab 20 with a stiffened perimeter 21, a plurality of radialreinforcing ribs 16 extending radially outwardly from the pedestal 10and a three-dimensional network of vertical 56, horizontal 110, 111,112, diagonal 58 b, 58 c, radial 58 and circumferential 59post-tensioning elements embedded in the footing that keeps all thestructural elements under heavy multi-axial post compression, whichreduces the stress amplitudes and deflections and allows the foundation100 to have a desirable combination of high stiffness and superiorfatigue resistance while improving heat dissipation conditions duringconstruction by having a small ratio of concrete mass to surface areathus eliminating the risk of thermal cracking due to the heat ofhydration.

A construction site is prepared by excavation, with flattening andpreparation of soil for the foundation 100. The foundation 100 may beset on pilings 400, on piers 180, or have anchors 404 (soil anchors orrock anchors 404 or micro-piles 401 or other types) in a conventionalmanner.

The present invention ensures good contact between foundation 100 andsoil, or sub-base 14 a, by casting.

The foundation 100 is cast against prepared soil, a crushed stonesub-base 14 a, a mud slab 14 or a membrane sheet in case of offshorefoundations assembled on a barge or in dry docks. Known grouting andleveling techniques under precast elements can be employed for ensuringplumb installation and good soil contact.

In one embodiment of the invention, the foundation 100 may be set on amud slab 14 or on compacted granular fill. The mud slab 14 is often athin plain concrete layer intended to provide a clean and level base forthe foundation installation. After the foundation site has beenprepared, a plurality of three or more precast stiffener ribs 16 areplaced on the mud slab 14 or compacted granular fill inside of theexcavation pit 12. The precast concrete stiffener ribs 16 may have meansfor leveling or other leveling techniques can be employed for level andplumb installation. If desired, grouting techniques can be used toensure complete rib base contact with the mud slab 14 or sub-base. Theprecast concrete ribs 16 may have bases 21 with left shear key 38 and/orshear connectors and right shear key 36 and/or shear connectors. Theprecast concrete stiffener ribs 16 may also have a vertical shear key34. The shear keys 34, 36 and 38 and associated dowels 40, 42 and 46 areto ensure continuous connections, with complete transfer of shear andbending loads, between the precast concrete rib stiffener 16 and thecast in place concrete which is to be poured into the foundation 100 toform the bottom support slab 20. The precast concrete stiffener ribs 16have upper dowels 40 and lower dowels 42 extending on the right and leftsides of the base 21 which have upper dowels 40 and lower dowels 42extending on the right and left sides of the base 21 which interconnectwith and spliced to upper mesh rebar 22 and lower mesh rebar 24installed between the ribs 16 and connected to dowels 40, 42 to formreinforcement for the slab 20 and pedestal 10 of foundation 100 when theconcrete is poured. The rib 16 has dowels 46 formed from rebar radiallyentering the pedestal 10 in the center of the foundation 100.

Doweling of rebar between the ribs 16 and foundation components can beachieved with rebar dowels extending from the prefabricated elements orby using rebar couplers, bar extenders or any mechanical rebar splicingsystem.

Arrays of grout or epoxy filled sleeves 42 b arranged in the slab 20could receive corresponding arrays of vertical dowels 42 a extendingfrom the bottom of prefabricated ribs 16 or perimeter beams 190 or otherprefabricated components.

Shear keys, or shear transfer mechanisms, can be replaced with, orcombined with, corbels or shear studs, or other shear connectors such asangled rebar or embedded steel shapes 34 a.

In another embodiment an array of steel beams 16 s, are encased into theweb of the rib 16 and extend inwardly into the pedestal cavity at theinner most end of ribs, and serve as a suitable shear force transfermechanism between the rib 16 and the pedestal 10.

In another embodiment, the foundation 100 comprises a steel frame fullyencased in concrete and has a central tower receiving a metal cylinder56 b fixed to an array of radially extending steel girders 16 s encasedin concrete beams 16, 21, 190 and rigidly connected at their outer endsto an array of perimeter beams 190 encased in the concrete foundation100 and a reinforced concrete slab-on-grade 20 covering the foot printof the foundation 100 and connected to the steel frame.

In one embodiment the ribs 16 may be treated with concrete bonding agentalong surfaces where cast in place concrete is received.

In another embodiment the foundation 100 may be provided with drains 23around the perimeter and the top surface of the slab 20 is slightlysloped towards the drains 23 such that water is drained away fromfoundation 100.

In another embodiment, when foundations are installed in sites prone toseismic activities and elevated water tables, the slab may have holes toprevent soil liquefaction during seismic events.

In another embodiment the ribs 16 or other foundation elements may becovered or coated with protective material for extending the life spanof the footing.

In another embodiment the ribs 16 are placed on the mud slab 14 firstand then the pedestal cage 50 made of an array of rebar, preferably Z orC shaped rebar and circumferential rebar is assembled around anchor boltassembly 60. Alternatively the pedestal cage 50 is assembled first or apreassembled pedestal cage 50 dropped into place first and then the ribs16 with dowels 46 are slid into place so that dowels 46 and shearconnectors 34, 34 a fit between the elements of pedestal cage 50 rebarassembly.

The precast concrete stiffener rib 16 has lifting lugs 32 to help placethe stiffener rib 16 into the excavated construction area. The base ofthe sibs may have a flat bottom surface such that the ribs may stand ontheir own on the mud slab 14 or compacted granular fill or duringtransportation from a precast plant to the foundation site. The precastconcrete stiffener ribs 16 have prestressing elements 58 running throughthe ribs 16 radially from the outside of the ribs 16 and throughpedestal 10. The radial prestressing elements 58 (or post tensioningelements) may be anchored to the opposite side of the pedestal 10 oroptionally run through the opposing precast concrete stiffener rib 16 onthe other side of the pedestal 10 and anchored at the end of theopposite rib 16. Once the ribs 16 and the pedestal cage 50 are in place,the dowels 46 extending radially inward from ribs 16 may be connectedto, or spliced with, corresponding dowels arranged in the pedestal cage50. Inside of pedestal cage 50 are additional rebar dowels 48 which willfacilitate the continuity of the structural components through thepedestal 10 as well as resist bearing, shear and bending loads.

Also inside of pedestal reinforcement cage 50 is a bolt assembly 60comprising a bolt template 52 an embedment ring 54 and anchor bolts 56protected by a PVC sleeve 57 or wrapped with a material to preventbonding between the anchor bolts 56 and concrete to be poured. Theanchor bolts 56 have a top portion which is used to attach the baseflange 301 a of a tower or column to the pedestal 10. A grout troughtemplate 52 at the bottom of the bolt template 52 may be used to createa grout trough 90 to ensure a good connection of the tower or column tothe pedestal 10. The grout trough 90 will be formed by removing the bolttemplate 52 from the anchor bolts 56 after the concrete has been poured.Radial dowels 46, prestressing elements 58 or shear connectors 34 at theinner end 16 c of ribs 16 should be spaced to clear anchor bolts 56 andother reinforcement arranged in pedestal cage 50.

In a preferred embodiment, for fully cast in place foundations, slabforms 17 may sit directly on the mud slab and rib forms 16 b aresupported and kept elevated above slab 20 elevation by means ofadjustable and reusable support legs 16 y arranged in the rib forms 16b. Small footings or thickened mud slab areas could be used under rib 16form support legs. Pedestal forms 102 can be supported by rib forms 16 bor by separate support legs.

When ribs 16 are prefabricated, the bolt assembly 60 is held in placeand the anchor bolts 56 are can be properly oriented by an alignmentapparatus 130. The alignment apparatus 130 has a central post 132 witharms 134 attached perpendicularly to the center post 132 and having legs136 for attachment to the top of the ribs 16 to provide added stabilityand proper bolt template 52 alignment during construction. The legs 136have an adjustable height relative to the arms 134. The arms 134 mayhave braces 138 attached to the central post 132 for holding the arms134 straight. The central post 132 may also have central post support135 to support the central post 132. The alignment apparatus 130 alsohas adjustable support members 140 for attachment between the arms 134and the bolt template 52 to align the anchor bolts 56 so they areupright. The alignment apparatus 130 can support the bolt assembly 60without the central post 132 by relying on the legs 136 supported byribs 16, which allows the lower portion of the central post 132 to beremoved if desired, Alignment apparatus 130 can be used as a template toensure proper location, elevation and orientation of ribs 16.

The ribs 16 can be of any shape or size depending on the specificationsof the tower and loads thereon. For example the ribs 16 may betrapezoidal, rectangular, T shaped or I beam shaped. The ribs 16 mayhave intermediate stiffener plates or diaphragms for improved structuralperformance. The ribs 16 or rib forms 16 b may receive ramps or catwalksthereon for easy access to the forms during construction.

Ribs 16, or rib forms 16 b, may have means for receiving and supportingperimeter forms 18, such as bolts or threaded inserts for receiving andsupporting the pedestal forms 102. The ribs 16, or rib forms 16 b, mayalso have attachment means 15 for holding base forms 17. The pedestalforms 102 may be equipped with platform sections for allowing accessaround the pedestal and the rest of the footing.

With all the rebar 22, 24, ribs 16, pedestal 10, bolt assembly frame 80and optional alignment apparatus 130 in place concrete forms may beattached such that concrete can be poured to form the pedestal 10 andslab 20 of the foundation 100. Pedestal forms 102 may attach to the ribs16, or rib forms 16 b, by bolts 18 or by any other means. Similarly, thebase perimeter forms 17 may be attached to the ribs 16, or rib forms 16b, by bolts 15 or by any other means. Alternatively, the base perimeterforms 17 may be supported to the ground or the mud slab.

With all the parts assembled all the rebar in place and the duct for theprestressing tendons, or prestressing elements of the foundation inplace, concrete is then poured into the pedestal 10 and between the ribs16. The pouring of the concrete can be accomplished quickly and the slabareas between the ribs 16 can be finished as the pedestal 10 concrete isstill being poured. The concrete may be used to build the pedestal 10and the slab 20 in one pour. Alternatively, the base for the entire slab20 foot print of the footing can be poured in a first pour then thepedestal 10 can be formed in a second pour.

When a bonded multi-strand post tensioning system is used in thefoundation 100, the prefabricated components are fitted with ducts andanchor hardware according to design specifications. The cast in placecomponents will be fitted with matching ducts to facilitate thecontinuity of tendons across the foundation 100. After the jacking oftendons, duct grouting is carried out as required. If the un-bonded,bundled mono-strand system is employed, no duct or grouting is required.

The structural load capacity of the foundation 100 is increasedsignificantly by the combination of radial 58 (or diametric) andcircumferential post tensioning 59. Circumferential post tensioning 59creates a desirable symmetric bi-axial post compression in the slab 20.Circumferential post tensioning 59 is applied at an elevation well belowthe neutral axes 16 n of the ribs 16 thus creating eccentric postcompression in the ribs 16 and the pedestal 10 and resulting inincreased nominal moment and shear capacity of the ribs 16 as well asimprovement in multi-axial fatigue resistant of the pedestal 10, ribs 16and the slab 20. Radial or diametric post tensioning elements 58 extendfrom rib 16 to opposite rib 16 across the pedestal 10. Radialpost-tensioning is applied with an eccentric load pattern, with higher.post compression below the neutral axis 16 n of the rib 16. When all theprestressing elements are jacked, the foundation 100 is kept under heavymulti-axial eccentric post compression stress, thus increasing rib 16structural capacity to resist soil support reaction and providing lowdeflections, high stiffness and low stress amplitudes resulting in highfatigue resistance and high durability of the slab 20. Backfill 13 isadded over the slab 20 for increased stability and stiffness of thefoundation 100.

After the concrete sets, post tensioning is carried out and thefoundation 100 is backfilled with compacted granular fill 13 tostabilize the foundation 100 against overturning.

Alternately, the bolt assembly 60 can be replaced by a tower section 56b embedded in pedestal 10 concrete. The embedded section 56 b havingmeans 56 c for receiving a tower base 301 by means of a boltedconnection arranged at the top of the section 56 b. The embedded metalcylindrical tower section 56 b encased in pedestal 10 concrete isprovided with holes 56 h for rebar and post tensioning tendons 58 toextend through the metal cylinder 56 b. Post tensioning 58 elements canextend through holes 56 h arranged in the cylinder and across thepedestal 10, through the ribs 16 to be anchored on distal ends of thefoundation.

Pedestal 10 can be any size or shape, round, triangular, square, polygonor other shape depending on the specifications of the tower and loadsthereon. The ribs 16 can be in any pattern around the pedestal 10. Inone embodiment shown in FIGS. 49, 50 the foundation 100 may have asquare pedestal 10 and ribs 16 at the corners parallel to the faces ofthe pedestal. The pedestal 10 may have a stepped construction with anenlarged lower cross section to reduce the length of the cantileveredribs 16.

Pre-assembled reinforcement sections (meshes) of the slab 20 componentscan be lowered into place in the slab 20 to speedup construction. Allrebar dowel or metal shear connectors extending through constructionjoints may be galvanized or Epoxy coated to prevent corrosion. The useof mechanical couplers in the foundation 100 may be limited or avoided.Specified mechanical couplers must be tested and certified for thenumber of load cycles in the life span of the foundation 100.

In another preferred embodiment, the ribs 16 are cast in place inreusable rib forms 16 b. The ribs 16 are cast in place jointly with thepedestal 10 in one continuous pour over the slab 20. Optionally, theribs 16, the pedestal 10 and the slab 20 are all jointly cast in onepour. All rib internal components including rebar assembly with dowelsand prestressing elements are placed inside the forms, then cast inplace concrete is poured into the rib forms 16 b as well as intopedestal 10 and slab 20 forms.

Rib reinforcing cages 16 rc can be assembled above grade and loweredinto the foundation in one or more sections.

In a preferred embodiment, rib forms 16 b with internal rib reinforcingcages 16 rc are preassembled and lowered into the foundation by cranesto mesh with slab reinforcing sections 22, 24 already placed in thefoundation. The radial reinforcing pattern 22 of the slab 20 enables themeshing rib dowels 42 between slab reinforcing 22, 24 without geometricinterference.

Ribs 16 can also be made in segments 16 sg and eventually united bymeans of doweling or by using segmented post-tensioned constructiontechniques. Rib anchor zone 16 x with anchor trumpets 16 t and hardwarecan be prefabricated separately of higher strength concrete than therest of the rib 16.

As shown in FIGS. 35 and 47, prefabricated perimeter beams 190 with posttension ducts 112 may serve as perimeter forms and become part of thestructure. An array of precast, rectangular or L-shaped beams 190 withmeans for connecting to the slab 20 and the ribs 16 can be used in thefoundation perimeter construction. The perimeter (edge) beams 190 canrest directly on the mud slab and connect to the slab 20 usinghorizontal dowels and shear keys arranged on the inner side. Optionallythe perimeter beam 190 is elevated and connects to the top of the slab20 using dowels 190 b extending from the bottom of the perimeter beams190 The precast perimeter beams 190 may have dowels 190 b and shear keys192 (such as corrugations) extending from their sides ends forconnecting to the ribs 16. In this case the ribs 16 will havecorresponding dowels 45 and shear keys 16 sh for receiving andsupporting perimeter beams 190. The connection between ribs 16 andperimeter beams 190 is established using closure pours in small cavitiesat the conjunctions of the ribs 16 and the perimeter beams 190.

The foundation 100 pertains to a hybrid gravity based and rock anchoredfoundation. Ribs 16 can be made with arrangement, mechanisms andconnecters for receiving piles 400 or micro-piles 401 or anchors 404 indifferent configurations. Vertical through holes 16 g in the ribs 16 canprovide means for receiving a pile 400, micro pile 401 or an anchor 404.Bearing elements 404 b and grouting are arranged on top of each rib 16to establish the required structural connection. An array of bearingplates 404 b with tensioning nuts 404 c on each soil/rock anchor may beused to compress the foundation 100 against supporting soil. Verticalthrough holes 16 g with corrugations 404 h for the anchor 404 extendthrough the foundation 100. Bearing plates 404 b with tensioning nuts404 c can be placed on top of the pedestal 10 or in the foundation 100.If desired ribs 16 may have piers 180 extending vertically from the ribs16 and the top of the pier elevation is raised to a higher elevation tomake anchor bolts 404 a accessible for tensioning and testing. Typicalrock or soil anchor construction and pouting methods can be utilized.Another option is to house rock anchor bolts 404 a and hearing plates404 b and tensioning nuts 404 c in accessible corrosion protectioncompartments above the foundation 100.

In another embodiment as shown in FIG. 43 and FIG. 44, the inventionpertains to a foundation 100 that comprises the following elements: 1 Avertically extending pedestal 10 that is cast in situ, out of concrete,the pedestal 10 serving to receive and support the tower structure; 2 Asubstantially horizontal support slab 20 that is cast in situ out ofconcrete, the support slab 20 covering an area of ground larger thanthat covered by the pedestal 10; 3 A plurality of radial ribs 16extending radially outwardly from the pedestal 10 and spaced around thepedestal 10, each rib being joined along the base thereof to the supportslab 20 and being joined along an inner side thereof to the pedestal 10,each rib has means for receiving a rock or soil anchor; 4 An optionalplurality of perimeter beams 190, or stiffened slab edge 21, spanningcontinuously, near the perimeter of the foundation 100 p, between ribs16 and supporting the slab 20 may be employed; 5 An array of soil orrock anchors 404 extending through the foundation 100, preferablythrough the ribs 16, may extend down into the ground below thefoundation, each anchor having a bearing element 404 b in or above thefoundation 100 and compressing the foundation against support soil whenthe anchors are tensioned, 6 Optionally the anchor can be grouted intothe ground to function as a pile anchor.

The prefabricated components can be molded at a facility undercontrolled conditions for good quality concrete setting and controlledrebar spacing which is superior to what can be obtained on a job siteand at a lower cost. The ribs 16, acting as deep stiff horizontalcantilever support, allow the base of the foundation slabs to have arelatively small thickness using less cast in place concrete and rebarthus lowering the cost for each foundation.

Alternatively, as shown in FIG. 36 ribs 16 may have reusable temporarysupports 16 y, or other means, arranged at the ribs 16 to hold the ribs16 in place, maintain them plumb during construction and elevate them ata predetermined height over slab reinforcing 22, 24. This style of ribs16 is intended to be raised above the ground or mud slab 14 so that thefoundation support slab 20 can be poured in place continuously underribs 16. Dowels 42 and shear connectors for this style may be arrangedat the bottom of the rib 16 for connecting with base slab 20 whichextends under the raised rib 16. When the concrete cures the continuoussupport slab 20, extending under the ribs, is united to theprefabricated ribs 16 and the ribs 16 are also united to the pedestal10. The rib inner ends 16 c will be partially encased in the pedestal 10to increase rib torsional end resistance. The final result is acontinuous monolithic foundation wherein loads are carried across thestructure vertically and laterally through the continuous structure bythe doweled and spliced reinforcing steel bars which are integrally castinto the pedestal 10, ribs 16 and support slab 20. The combination ofthe high stiffness of the ribs 16, solid pedestal 10 and continuous slab20 construction across the pedestal 10, and under ribs 16, allows theslab 20 to behave structurally as a continuous slab 20 over multiplerigid supports resulting in small flexural and shear stresses in theslab 20, reducing deflections, improving fatigue conditions andincreasing the stiffness of the foundation as well as allowing for thebenefits of an economical design.

Cast in situ concrete can be shielded from extreme weather, includingheat, cold, rain and snow, by simply extending blankets, covers orshields between ribs 16 during construction, and then using heaters orfans as required to regulate the temperature and humidity of theconcrete to allow for proper setting and curing conditions.

Another embodiment of the present invention pertains to a levelingtechnique that simplifies the tower base leveling process and shortensthe number of steps required for grouting under a tower base. The bolttemplate 52 is provided at the very top of the bolt assembly 60 with atleast three sets of additional bolts 53 and corresponding threaded boltinserts 53 h suitable for embedment in the concrete. Such leveling bolts53 and inserts 53 b will be located outside or inside the bolt circle 60a of tower base, but directly under tower base flange 301 a. This allowsfor continuity of grout bed 90 a construction and provides an easyaccess to leveling bolts 53. Small cutouts at leveling bolt locationsmay be used. Another benefit of this leveling technique is having theability to apply continuous grout bed 90 a that is free of cold joints,under the tower base flange 301 a in one session as well as having theability to tension all anchor bolts 56 in one work session.

In another embodiment the onshore foundation may have a pedestal 10 thatis rigidly connected to vertical concrete stem 11 that is fixed to atower base of a wind tower. The pedestal and the stem are verticallyprestressed with vertical post tensioning elements extending through theheight of the foundation. The stem is fitted with an array of bolts 60for receiving and supporting the tower base 301.

The foundation design, as shown in FIG. 44, can be reconfigured tosupport lattice towers 200 comprising multiple columns connections tofoundations in a spaced array. The ribs 16 will be provided with columnreceiving components including embedded anchor bolts (or grouting aroundan embedded element) and an integral pier design into the rib 16. Therib geometry may be widened and enlarged at the integral pier 180. Thearray of said integrated piers ribs 16 are fitted with means forreceiving and supporting the legs or the columns 200 a of the latticetower 200.

The integral piers 180 can extend above final grade elevation, while thetop of pedestal 10 may stay below final grade elevation. For thisfoundation style, pedestal elevation may be depressed and towerreceiving components may not be required in the pedestal 10. Thisconfiguration may also be used in offshore applications wherein aprefabricated gravity foundation 100 is connected to lattice towerstructure 200 that is fitted with a wind tower receiving component atits top. The foundation 100 will be installed over prepared seabed andfilled with a suitable backfilling material 13, and surrounded withscour protection 13 b.

As shown in FIG. 45, in permafrost condition, the foundation 100 may besupported on an array of concrete piers deeply embedded and frozen intothe ground. Anchor bots 404 a can be used to secure the ribs 16 to theirsupporting piers around the perimeter of the foundation 100 p. The slab20 bottom elevation set above grade elevation. Alternatively, the slabmay not be used in the design.

In another embodiment, as shown in FIG. 1, the invention pertains to afatigue resistant gravity based spread footing for use under heavymulti-axial cyclical loading of a wind tower 300 which comprises aplurality of components, namely a central vertical pedestal 10, asubstantially horizontal continuous bottom support slab 20 withstiffened perimeter 21, a plurality of radial reinforcing ribs 16extending radially outwardly from the pedestal 10 and athree-dimensional network of vertical 60, horizontal 110, 111, 112, 58,diagonal 58 b, 58 c, radial (or diametric) and circumferential 59post-tensioning elements that keep the structural elements under heavymulti-axial post compression with specific eccentricities andorientations that are intended to reduces stress amplitudes anddeflections and allows the foundation 100 to have a desirablecombination of high stiffness and superior fatigue resistance whileimproving heat dissipation conditions during construction by having asmall ratio of concrete mass to surface area thus eliminating the riskof thermal cracking due to heat of hydration.

Vertical prestressing of the pedestal 10 can be carried outindependently of tower receiving elements. A pedestal 10 may have anarray of vertical post tensioning elements 56 that does not connect to atower 300, and an embedded tower section 56 b bolted to a towerstructure 300.

Radial post-tensioning 58, extending across the foundation 100, in pairsof ribs 16, allows for the desirable structural continuity and thedirect transfer of loads from downwind ribs 16 into the pedestal 10 andthen into the opposing upwind ribs 16. Radial and circumferential postcompression stresses in the slab 20 and/or perimeter beams 190 allowsfor a desirable reduction in stress amplitudes the structural continuitybetween slab 20 spans and/or perimeter beam 190 spans, across the ribs16, thus creating a desirable load sharing mechanism between adjacentribs 16 by forcing more ribs 16 to be engaged in resisting tower loads.

The invention pertains to a durable, high-stiffness, fatigue-resistantfoundation structure 100 for onshore wind tower installations whichcomprises: 1. a central pedestal 10 that is made of cast-in-placeconcrete with concentric vertical prestressing elements 56, 70 andeccentric multi-axial horizontal and/or radial post-tensioning elements58 a, 58 b, 58 c; 2. an array of cast-in-place eccentricallypost-tensioned radial ribs 16; 3. a cast-in-place slab 20 with heavilypost-tensioned thickened slab edge 21.

All components are made of high strength reinforced concrete and arerigidly connected to each other to behave as a monolithic spreadfoundation structure. The structural components are rigidly connectedwith arrays of rebar dowels 42, 46 (passive reinforcing) and/orpost-tensioning elements extending through the conjunctions. The slab 20functions as a two-way slab system that is free of construction jointsacross the footprint of the foundation and spans continuously overmultiple ribs 16. Perimeter post tensioning 59 a or circumferential posttensioning 59 of the slab 20 is applied at an elevation well below theneutral axes 16 n of the ribs 16 to cause eccentric loading of the ribs16 and the pedestal 10. Radial post-tensioning elements 58 with aneccentric load pattern, with higher post compression at the bottom ofthe rib, extend from rib end 16 x to the opposite rib end 16 x acrossthe pedestal 10, or to the opposite end of the pedestal 10. When all theprestressing elements are jacked, the foundation 100 is kept under heavymulti-axial eccentric post compression stress, thus increasing rib 16structural capacity to resist soil support reaction and providing lowdeflections, high stiffness and low stress amplitudes resulting in highfatigue resistant and high durability. Backfill 13 is added over theslab 20 for increased stability and stiffness of the foundation 100.

Soil support reaction under the slab 20 is transferred from the slab 20to the ribs 16 and thickened slab edge 21 (or perimeter beams 190) as intwo-way slab systems with more load distribution going to the ribs 16 inthe primary span. Perimeter 112 or circumferential 59 post-tensioning isapplied, generally in the orientation of the primary span thateffectively reduces stress amplitudes and deflections in the slab 20 bykeeping the slab 20 under heavy post-compression in the directions ofprimary slab spans 20 s 1, and secondary slab span 20 s 2 around thefoundation. The size, distribution, eccentricity and location of posttensioning elements 58 in the ribs 16 and the slab 20 are used todictate the natural frequencies of the foundation 100 to be in a saferange relative to operating frequencies of the wind generator accordingto turbine manufacturer recommendations.

The 3-dimensional post-tensioning network in the foundation keep all thestructural components (Pedestal 10, ribs 16, slab 20, thickened slabedges 21 (or integral edge beams)) under multi-axial post compressionconfinement resulting in lower stress range amplitudes thus yieldinghigher stiffness, more effective crack control, lower deflections andimproved fatigue resistance. Superior fatigue resistance and longlifespan are achieved by keeping most of the structural elements of thefoundation 100 under multi-axial compression while resisting operatingloads or even during normal and abnormal extreme loads from thesupported structure (wind power generator).

In a preferred embodiment, rib post-tensioning requirements are reducedby engaging fully developed rebar dowels 46 from the rib 16 into thepedestal connection as well as extending fully developed radial rebardowels 22 r, 24 r of the slab 20 into the pedestal 10, thus allowingpassive reinforcing to participate in the connection especially underextreme loads. A radial slab reinforcing pattern with tapered rib widthis very cost effective as the rib 16 to pedestal 10 connection benefitsfrom a large number of top and bottom radial slab reinforcing bars 22,24 participating in said connection, as the rib width widens, thusreducing the number of bottom post-tensioning strands 58 a required forthe connection.

The structural configuration of the foundation 100 reduces the overallcumulative deflections in the structure under tower loads andsignificantly improves the rotational stiffness of the foundation 100which is a key factor in determining the size of foundations in windturbine installations. The rotational stiffness is also improved by theinterlocking between the surrounding soil (after backfilling) and themultiple surfaces and vertical faces of the foundation structure. Thehorizontal stiffness is improved by the passive earth pressure on themultiple surfaces of the structure. Both rotational and horizontalstiffness achieved by this design are much higher than conventionaltapered inverted-T gravity spread footings especially for onshorefoundations installed below grade in an excavated pit because of theincreased interlocking surface area and increased passive earth pressureand increased friction on the multiple faces of the fatigue resistantfoundation 100.

The solid-core pedestal 10 comprises a continuous reinforcing cage 50and a tower receiving component &6, such as anchor-bolt assembly 60,with a cylindrical array of bond protected high strength post-tensioningbolts 56 for connecting to wind tower base flange 301 a. In anotherembodiment and the tower receiving component may comprise an embeddedcylindrical metal tower section 56 b with means 56 c for connecting to atower section such as a flange 56 c with bolt holes 56 d for receivingbolts 301 b at its top and with an array of holes 56 h to allow thepassing of rebar dowels 46 and post tension tendons 58. The embeddedtower section 56 b is also fitted with conventional bearing flanges 56 eand ring stiffeners for interlocking with the pedestal concrete. Theanchor bolt assembly 60 ensures structural continuity between the tower300 and the pedestal 10. The post-tensioning forces of the anchor bolts56 are selected to insure that the tower base flange 301 a remains incontact with the pedestal 10 under extreme normal and abnormal loadconditions. The bolt assembly 60 includes, at its bottom end, a bearingelement 54 that may consist of an embedment ring plate 54 that is madeof segments that are welded together.

As shown in FIGS. 7 and 26, radial post-tension tendons 58 and rebardowel reinforcing elements 46 extending from the ribs 16 and the slab 20pass through the pedestal reinforcing cage 50, or through holes 56 h inthe embedded metal tower section 56 b.

As shown in FIG. 17-22, post-tensioning elements 58, 58 a, 58 b, 58 care flared horizontally, profiled vertically, arranged in matrix groups,spaced and draped in a manner that allows for optimum utilization ofpost-tensioning and ease of installation while avoiding tendoncongestion and stress concentrations as tendons 58, 58 a, 58 b, 58 ccrisscross in the pedestal 10. The regrouping of tendons to form a flatand wide matrix along each axis was found to be effective in avoidingtendon congestion especially in the pedestal 10. The flat and widematrix of tendons are placed as high or as low as possible to maximizetheir moment arms and optimize their contributed moment capacity. Forcorrosion protection, bonded (multi-strand and grouted) or un-bondedencapsulated (mono-strand) post-tensioning elements and their associatedconstruction techniques can be used in the foundation 100.

The rib's thickness 16 th can be gradually increased at the connectionto the pedestal 10 to increase rib flexural, shear and torsionalcapacity and enhance pedestal confinement 16 m. The post-tensioningrequirements can be reduced by engaging dowels 46 at the rib-to-pedestalconnection and by extending fully developed radial dowels 46, 22 r, 24 rfrom the rib 16 and the slab 20 deep into the pedestal 10, thus allowingpassive reinforcing to participate in the connection.

In another embodiment, as shown in FIG. 2, ribs 16 top surface can betapered to a substantial slope extending vertically to an elevation nearthe top of pedestal 10 allowing the ribs 16 to benefit from diaphragmaction at their inner zone and also provide lateral support for the fullheight of the pedestal 10 and to provide concrete confinement at thehighly stressed zone at the top of pedestal 10 under tower base flange301.

The foundation may have a circular or polygonal foot print. Thethickened slab edge 21 (or perimeter beam 190 may extend above or belowthe foundation. A shallow perimeter beam 190 profile should be selectedfor ease of backfilling and improved accessibility for roller compactorsduring the backfilling of the foundation 100. A thickened slab ring beam21 may be designed to be at an offset distance away from the slab edgeallowing the slab segment, outside the ring, to behave as a cantilever.This configuration reduces slab 20 span and deflections as well as thevolume of concrete required in the foundation 100.

As shown in FIG. 5, the configuration of the slab 20 and its continuousreinforcing including that of the thickened slab ring beam 21 isconfigured to create a rigid composite connection to the ribs 16 withhigh stiffness which is sufficient to allow adjoining ribs 16 toparticipate more in resisting the loads and thus reducing localdeflections and increasing overall foundation stiffness in addition toreducing the unsupported length of cantilever radial ribs 16.

In a preferred embodiment, as shown in FIG. 13, the pairing of the ribs16 on distal ends 10 x and the continuous perimeter beam 21 constructionyield a cost effective layout of post-tensioning that uses a smallnumber of tendons and corresponding anchors 59 b as well as reducesfriction losses by avoiding sharp turns in tendon layout. The tendons 58of the ribs 16 are anchored in a matrix array 58 m at the outer end ofthe rib 16 and extend horizontally and diagonally along the rib 16 tosplit into at least two groups 58 a and 58 b one near the bottom and theother near the top of the rib as it connects to the pedestal 10. Thetendons 58 are more concentrated at the bottom than at the top in aconcentric prestressing pattern 58 m 1 that is intended to maximize thestructural capacity of the foundation and meet the flexure and sheardemand of the governing load cases.

Ribs 16 may have thickened flanges, at their connection to the pedestal10 that may also house post tensioning anchors for tendons 58 extendingfrom ribs 16 on the opposite side of pedestal 10. The ribs 16 may alsohave post tensioning anchors along their sides or tops if tendoncurtailment methods are applied in the design. The ribs 16 may also haveembedded loop anchors if looping of tendons is used in the design. Loopanchors 70 could also be used in the pedestal 10 to support andvertically prestress precast concrete towers 300 b, or concrete stems11.

As shown in FIG. 21, the tendons 58 in ribs 16 extend horizontally anddiagonally to be split into three distinctive groups as they enter thepedestal 10. The first group 58 a with more tendons is placed at thebottom of ribs 16 or in the slab 10 to create camber for reducingdeflections and improving foundation soil contact as well as meet thehigh flexural demand from the governing load cases, and the second group58 b has a slope up diagonally to follow the geometry of the top of therib as they enter the pedestal 10. The third group 58 c is in the middleand it starts horizontal at rib anchor block 16 an and diagonally slopesdown towards the bottom of the rib 16 to enter the pedestal 10 foroptimum use of the tendons 58. Tendons 58 in the pedestal 10 are fannedand flared into groups in a flat pattern 58 m 2 to simplify theinstallation and maximize their utilization by increasing theireffective depth or moment arms measured from the top or the bottom ofthe structural concrete. Additional post-tensioning groups for shearresistance can be provided by providing tendons 58 that traverse theshear failure plane in the ribs 16.

In another embodiment, as shown in FIG. 21, the post-tensioning in theribs 16 consist of three distinctive groups: 1. A bottom group oftendons 58 a that is horizontal at the bottom of the rib 16 and in theslab 20 and may be grouped with slab post tensioning, 2. A top group oftendons 58 b that is diagonally sloped upward to follow the geometry ofthe rib top, 3. An optional middle group of tendons 58 c that startshorizontal at rib outer edge 16 x and is diagonally sloped down towardsthe bottom of the rib 16 to eliminate dead load deflections and keep theribs 16 and pedestal 10 under post compression during normal operatingconditions and also provide the high demand of post-tensioning capacityrequired at the bottom of the rib 16 for downwind load cases, andtraverse the shear failure plane for ribs 16 in the governing downwindload cases and provide additional shear resisting capacity in each rib16, such that the number of strands in the bottom of the rib 16 and thepedestal 10 is much higher than that at the top thus causing amulti-axial, heavy, eccentric horizontal post compression in thefoundation after all the tendons 58 are jacked.

Alternately, as shown in FIG. 13, anchor-blocks for perimeter orcircumferential post-tension tendons can be placed at perimeter beams190, (ring beams 21) at the thickened slab, at the edge of thefoundation on top of perimeter beams 190 or on the sides of ribs 16. Apreferred layout with two anchor blocks 21 a on opposite sides of thefoundation and with a semi-circular (180-degree) tendon arrangement isshown in FIG. 13. Ring tendons 59 with ring anchors 59 b (such asdog-bone anchors) can be used, with perimeter or circumferentialtendons, to avoid having blisters on the foundation 100. Styrofoamblock-outs 53 a can be placed in the foundation 100 according to anchormanufacturer recommended dimensions. When the concrete reaches thesufficient strength ring tendons 58 are jacked and ring anchors 59 bgrouted.

In another embodiment circumferential post tensioning may be made withmultiple tiers of tendons 59, in this case anchor block 21 a locationsor ring anchor 59 b locations for each tier may be staggered around theperimeter of the foundation 100 p to reduce stress concentration.Corrosion protection must be provided at anchor locations. Perimeterpost-tensioning 112, or circumferential post-tensioning 59 can be madewith bundled, un-bonded mono-strands without encapsulation.

The foundation may be made with a network of prestressed concreteelements that can be structurally analyzed, with the strut and tiemethod. A three-dimensional structure made of an array of vertically andhorizontally oriented truss-girders joined at the center may be used,with major tension chords reinforced with prestressing tendons, based onboth upwind and downwind load cases. The tension forces in the structureare resisted largely by prestressing elements and passive reinforcing.Compression forces are resisted largely by the concrete elements. Thestructure can be analyzed as a circumferential array of verticallyoriented trusses that are fixed at their inner ends 16 c to the centralpedestal 10 and are laterally stabilized at their bottom by a horizontaltrussed diaphragm formed by perimeter post tensioning 59 a, in the slab20 or perimeter beam 190, and radial bottom tendons 58 in the ribs 16 orthe slab 20.

In another embodiment, as shown in FIG. 1, the fatigue resistantfoundation 100 comprises a circumferential array of vertically orientedeccentrically prestressed cantilevered girders 16 that are fixed attheir inner ends 16 c to a central pedestal 10 that is laterallysupported and confined through most of its height by rib concrete, andthe ribs 16 and pedestal 10 are laterally stabilized at their bottom bya horizontal prestressed concrete trussed diaphragm, with a continuousslab 20, and the prestressing is provided by radial tendons 58 in theribs 16 (or the slab 20) and circumferential post tensioning elements59. The radial 58, and circumferential 59 tendons provide eccentricprestressing in the ribs 16 and the pedestal 10. The pedestal 10 isvertically prestressed by an array of vertically extending anchor boltcircle 60 and is structurally fixed to a tower base 301 of a pylon. Theslab is prestressed with tendons 110, 111 and 112 and circumferentialtendons 59.

In another preferred embodiment the construction of the foundation 100may utilize pre-assembled slab perimeter reinforcing cages 21 c, builtin segments with overlapping spliced bars at their ends, and each havingan array of shear resisting vertical ties 21 vt and flexure resistinghorizontal bars 21 h as well as anchor zone reinforcing. Slab perimetercages 21 c or perimeter beam reinforcing cages, 190 c can bepreassembled and then placed in the foundation.

As shown in FIGS. 4, 5 and 6, the foundation has specific reinforcinggroups. The ribs 16 have flexure reinforcing tendons 58 concentrated atthe bottom and the top, vertical stirrups 16 vt for shear reinforcingtightly spaced in high shear zones along rib inner end 16 c, rib skinreinforcing on each face 16 fs and bursting and splitting reinforcingmade of horizontal hairpins 16 hp extending between the rib skinreinforcing 16 fs, as well as straight, hooked or U-shaped horizontaldowels 46 for embedment into the pedestal 10 and vertical dowels 42, atthe bottom of the ribs 16 are used, for composite action with the slab20. As shown in FIG. 4, the vertical stirrups 16 vt also function asdowels for composite action of the slab 20. The dowels may be spacedsuch that they mesh between slab reinforcing bars without geometricinterference. The rib reinforcing 16 re is built in preassembled cagesand placed over the slab reinforcing 22, 24. In order to maximize shearcapacity vertical stirrups 16 vt are placed side-by-side, in pairs, atthe inner rib zone 16 c where the shear demand is high.

Anchor zones, as shown in FIGS. 5, 6, 21, 22, are provided with heavyreinforcing with trim bar and ties 16 tt as well as surface reinforcingat the anchor block location. The ribs 16 may also have horizontalreinforcing dowels 45, perpendicular to the ribs 16, to facilitate thestructural continuity of the supported perimeter beams 190 or thethickened slab 21, across the width of the rib 16, by means of splicingthe dowels 45 with perimeter reinforcing 21 c, 190 c.

The pedestal 10, as shown in FIG. 11 and FIG. 12, has a horizontal mesh50 t at the top and skin reinforcing 50 c 1 at all surfaces as well asat least one cage 50, around the anchor bolt assembly 60, comprisingvertical tightly meshed bursting reinforcing 50 c including twocylindrical meshes 50 c 1 & 50 c 2 confining the anchor bolts 56 eachcomprising horizontal hoops 50 c 1 h & 50 c 2 h and either C or Z-Shapedbars 50 cv and a radial array of horizontal hair-pins 16 hp or stirrupstying both cylindrical meshes 50 c 1 & 50 c 2 or spiral stirrups eachhousing a number of anchor bolts 56. The pedestal 10 cage assembly maycomprise two concentric tightly meshed cages 50 c 1 & 50 c 2 surroundingthe anchor bolts 56 one from the inside and the other from the outsidewith a radial array of bursting and splitting resistant hairpins 16 hpextending between the two cages 50 c 1 and 50 c 2. Additionally, anarray of vertically oriented pedestal 10 vertical bursting out of planestress resistant reinforcing group of reinforcing elements, comprisingcircumferentially spaced vertical hairpins 50 vt extending between saidtop horizontal mesh 50 t and a horizontal bottom reinforcing mesh 50 bin the pedestal 10 or slab 20, is included in the pedestal cage 50. Thevertical hairpins 50 vt in pedestal core 10 a also function as supportsto secure tendons in the pedestal 10 during construction.

Upper 22 and lower 24 slab reinforcing meshes may have any pattern suchas a square grid, a circular array with radial pattern or overlappingpie-shaped segments. Additionally, there may be an array of slabreinforcing 22, 24 locally arranged beneath the ribs 16 orientedparallel to the ribs 16 and extending into the pedestal 10 to facilitatecomposite action. The slab 20 may also be reinforced withpost-tensioning elements in any pattern including radial,circumferential, perimeter or a square grid.

The foundation may utilize many prefabricated components including rebarmeshes and cages, pedestal cage assembly, pre-cut post-tensioningstrands, preassembled post-tensioning bundles, pre-cut post-tensioningduct sections and prefabricated concrete forms.

Reusable rib forms 16 b may be utilized to form the foundation perimeter100 p, the ribs 16 and the pedestal 10. Forms can be made to besegmented, universal, expandable and adjustable to work for differentfoundation sizes. Rib forms 16 b can be made with adjustable supports 16y to elevate the forms above the wet slab 20 concrete duringconstruction if the foundation is built in one pour. Rib forms 16 b maysit directly on the hardened concrete slab 20 if the foundation is builtin two pours. Rib forms 16 b may be made with two side-panels ofstiffened non-stick plates and an array of adjustable horizontal spacersbetween the panels to maintain proper geometry and resist the lateralpressure of wet concrete. Rib forms 16 b and pedestal forms 102 may befitted with lifting lugs 32 or means for receiving and supportingladders, catwalks 95 and work platforms 95 to allow for access aroundthe foundation 100. The forms may have means for securing post-tensionanchors and hardware at specific spacing during construction. The formsmay also have means for hanging and supporting rib reinforcing cages.

The foundation 100 may be supported on piles, or micro-piles or piers orrammed-aggregate piers. The foundation 100 may receive rock (or soil)anchors 404 in a conventional manner.

A construction site is prepared by excavation, grading and compactionsoil for the foundation. The foundation 100 may be set on a mud slab 14or on compacted granular fill. The mud slab 14 is a thin plain concretelayer intended to provide a clean and level base for foundationinstallation.

In one embodiment, as shown in FIG. 48 after the foundation site hasbeen prepared, the slab reinforcing 22, 24 is placed inside slab forms17 and the slab 20 is poured in place with dowels 42 extending up fromthe slab 20 to receive the ribs 16 and the pedestal 10 in a second pour.The rib rebar 16 fs and pedestal rebar 50 and cage 60 placement withpost-tension tendons 58 (or duct) placement are set in place rib forms16 b and pedestal forms 102 are installed before a second pour iscarried out. Alternatively, the foundation 100 can be poured in a singlepour with the use of accelerators in the concrete mix and by following awell-designed concrete pour sequence. A set of small footings 16 f,placed within the mud slab, can be used to support and elevate the ribforms 16 b and pedestal forms 102 during construction. Slab 20, pedestal10 and rib 16 reinforcing elements are assembled in the foundation 100.Forms are placed in the foundation around the perimeter, the ribs 16 andthe pedestal 10 and the concrete is poured into the foundation 100 in acarefully designed pour sequence. One option is to start with slab 20and the bottom part of the ribs 16 and the pedestal 10 with acceleratorin the concrete mix to seal the bottom of rib 16 and pedestal forms 102by the time the slab 20 concrete is finished, the ribs 16 and thepedestal 10 are poured jointly in small lifts.

When the concrete hardens to a specific strength, the post-tensionelements are jacked and grouted as required. The tower base flange 301is then attached to the pedestal 10 and grouted, and the tower anchorbolts 56 are tensioned after the grout reaches sufficient strength.

In a preferred embodiment, as shown in FIG. 13, the invention relates toa high stiffness, fatigue resistant, wind turbine foundation 100,supporting a wind generator with a multi-megawatt rating and subjectedto extremely high cyclical upset Loads that comprise the followingcomponents comprising: 1. a substantially wide central pedestal 10 withsubstantially solid core concrete construction 10 a that is kept,through most of its height, under a combination of lateral structuralconcrete ribs 16 and confinement: 16 m, high vertical post.-compressionstress and high eccentric multi-axial lateral horizontalpost-compression stress across its width, provided by said lateral ribs16 and post-tensioning elements 58 that traverse the width of thepedestal 10, through non-segmented concrete construction, along multipleaxes in a concentric pattern, and having a set of upright,circumferentially spaced anchor bolts 56, for providing the highvertical post-compression stress, extending through said pedestal 10,and having lower ends anchored to an anchor ring and upper endsprojecting upwardly from said top end of said pedestal, said anchorbolts 56 being substantially bond protected along their length, saidupper ends of said bolts 56 project upwardly from the pedestal 10through a base flange 301 a of an annular tower 300 structurally fixedatop the pedestal 10, and also having an upright heavily reinforced cageof tightly meshed rebar, and concentrically arranged around both sidesof the anchor bolt cage with an opening to allow the passing of lateralload transfer elements 58, 46, 2. a support slab-on-grade 20,cast-in-situ out of concrete against the soil, in an excavation pit 12,of continuous construction and covering a footprint substantially largerthan that of the pedestal 10 and having a thickness that is much smallerthan the depth of the pedestal 10 and having a thickened edge 21 made ofconcrete integral with the support slab 20 and having horizontalpost-tensioning elements 58, 59, 110, 111, 112 to keep the slab 20 underheavy multi-axial post compression, 3. an array of concentricallyarranged ribs 16 made of deep girder construction, integral with thepedestal and support slab 20, and jointly east-in-situ with saidpedestal 10, and extending vertically, above the slab 20, to anelevation near the top of pedestal 10 such that the pedestal 10 islaterally supported and substantially confined below the tower baseflange 301, the ribs 16 having a width that is substantially smallerthan that of the pedestal 10, and being arranged such that pairs of ribs16 outwardly extend from opposite sides of the pedestal withpost-tensioning elements 58 inwardly extending from the distal ends 16 xof the ribs 16 through the pedestal 10, 4. reinforcing prestressed rebardowels 46 extending from the ribs 16 deep into the core of the pedestal10 from distal ends 10 x, and arrays of dowels 42, 46, made of rebar,extend between the slab 20 and each of the ribs 16 and the pedestal 10along their conjunctions, 5. a suitable backfill material 13 placed overthe slab 20, to stabilize the foundation 100 against overturning,followed by tower base installation and grouting, the foundation 100 iskept under heavy multi-axial post-compression such that tower loads areresisted by pairs of ribs 16, on distal ends 10 x of the pedestal 10,wherein each pair of ribs 16 form a high stiffness continuous,non-segmented, laterally supported, post-tensioned girder extendingbetween distal ends of the foundation 100 with continuous uninterruptedcomposite action from the slab-on-grade 20.

In another embodiment, slab post-tensioning can be arranged at anycombination of perimeter, radial, diametric, or other patterns.

In another embodiment, composite action is further facilitated withradially oriented, reinforcing bars 24 r 1 locally arranged in the slab20, beneath the ribs 16, and extended deep into the pedestal 10, inaddition to an array of vertical dowels 42 extending between the rib andthe slab 20 that function as shear connectors.

In a preferred embodiment, as shown in FIG. 13, the invention pertainsto a foundation 100 for supporting a wind generator with amulti-megawatt rating and subjected to extremely high cyclical upsetloads, with increased stiffness and improved fatigue resistantcomprising: 1. a support slab-on-grade 20 of non-segmented continuousconstruction with a circular integral perimeter beam 190 withcircumferential post tensioning elements 59 made of two 180-degreetendon segments forming a 360-degree circle, with anchors 59 b at theopposite sides of the foundation, 2. a central cylindrical pedestal 10integral with the support slab-on-grade 20 of solid non-segmentedconstruction and having vertical post-tensioning elements, 56, 3. ribs16 integral with the support slab 20 and the central pedestal 10, on topof the slab 20, with three or four pairs of ribs 16 radially extendingfrom opposite sides of the pedestal 10 and post tensioning elements 58extending axially and diagonally from anchors 16 an placed at the distalends 16 x of the ribs 16 through the pedestal 10, such that the ribs 16and the perimeter beams 190 function as a prestressed trussed diaphragmstructure with the slab 20 acting as infill panels, and pairs of ribs 16on distal ends 10 x of the pedestal 10 function as continuouspost-tensioned girders, that are free of construction joints, withcontinuous composite action from the slab 20 and the foundation 100 iskept under eccentric multi-axial horizontal and concentric verticalpost-compression, with circumferential post-tensioning 112 in the slab20 which effectively reduces stress amplitudes and deflections in theslab 20 by keeping the slab 20 under heavy post-compression in thedirection of the primary slab spans 20 s 1 which is in a radialorientation.

In a preferred embodiment, the rib 16 extends vertically from the bottomof the foundation 100 to an elevation near the bottom of the tower baseflange 301 to enable the ribs 16 to participate in resisting bearingloads under the tower base flange 301 by increasing the area of thecross-section involved in bearing resistance under the tower base flange301 and increasing the permissible bearing strength under the baseflange 301 or the grout bed 90 a and by increasing the bearing areameasured at the surrounding faces of the concrete. The geometricconfiguration and the improvement in bearing resistance, allow concretewith only one relatively low compressive-strength for the entirefoundation structure. In contrast, high bearing stresses under the towerbase flange 301 in conventional gravity spread footings, requiresconcrete with higher compressive strength for the pedestal 10 and alower compressive strength for the slab 20.

The proximity of inner rib ends 16 c to the tower base flange 301 allowsthe inner zones of the ribs 16 to remain under vertical compressionstresses caused by vertical post-tensioning forces between embedmentring 54 and tower base flange 301. The vertical compression stress zonesin the distal ends of the pedestal 10 x improves the confinementconditions and fatigue resistance in the rib inner zones 16 c.

Bonded and grouted multi-strand in some applications may be tooexpensive and take too long to install as it requires an additional stepof grouting and may not be economical for some onshore installations. Itmay then be preferable to use un-bonded, encapsulated mono-strands,arranged in bundles and installed in the foundation reinforcing prior toconcrete casting, which reduces construction costs and improves theconstruction schedule.

In a preferred embodiment post-tensioning in the foundation 100 is madeeccentric, to create cambers in the foundation 100 that could result inreduced deflections and improved foundation-soil contact. As an example,the eccentric prestressing of the ribs 16 creates a convex shaped camberin the foundation 100 that helps reduce the deflections under turbineweight and operating loads. Similarly, cambers can be used in perimeterbeams 190 and slab sections to reduce slab deflections and improvefoundation-soil contact conditions by ensuring a more uniform bearingpressure under the foundation thus allowing for an optimized foundationfootprint with more uniform pressure over the effective bearing area.

The vertical profile (elevation) of circumferential tendons 59 in thefoundation 100 may be varied at mid spans and under supporting ribs 16to optimize their utilization.

In another embodiment a gradual transition of geometry at theconjunction of the structural elements is employed to prevent stressconcentration and fatigue related problems. As an example, the use offillets and curved transition ft is desirable at the conjunctionsbetween ribs 16, pedestal 10 and the slab 20.

In a preferred embodiment, the inner ends of the ribs 16 are tapered toa wider cross-section as the rib 16 connects to the pedestal 10, inorder to satisfy the high flexural, torsional and shear demands at theinner zone of the ribs 16, and to distribute the multi-axial compressionover large surface area to help reduce splitting and burstingreinforcing on the side of the pedestal 10.

In another embodiment low relaxation post-tensioning strands are used toreduce post tension losses over time. Concrete accelerators andplasticizers and other admixtures may be utilized in the concrete mixdesign. The small thickness of the structural elements may allow foron-site steam curing of the concrete.

A hollow pedestal 10 cross-section may be used, however it can beproblematic. A hollow pedestal above the frost depth where there iselevated water table may be problematic. In another embodiment thecross-section of the rib may change and dimensions along its length maychange. For example, the section may start rectangular and gradually atop flange may be enlarged to reduce stresses in the upper zone of therib.

In another embodiment the pedestal 10 may have an enlarged cross-sectionat the top followed by a transition into a smaller cross-section below.The upper enlarged cross-section may help improve hearing strength atthe top of the pedestal below the tower base flange 301 a, the bearingwasher plate 404 b, and the high strength grout bed 90 a according toAmerican Concrete Institute design guidelines.

The present invention pertains to a foundation design that overcomes thethermal cracking problem stemming from heat of hydration, in largefoundation pours, by using a structural configuration coupled withpost-tensioning techniques that reduce the thickness of the structuralelements, while increasing the surface area of the concrete pour, thusimproving heat dissipation conditions and causing a the ratio ofconcrete mass to surface area to be roughly 40% to 50% less than inconventional design for inverted T foundations for the same turbineunder the same loading and geotechnical conditions.

As shown in FIG. 37, FIG. 38 and FIG. 39, a tower base leveling andgrouting method can be used which does not employ tower anchor bolts forleveling, or leveling shims which cause undesirable stress concentrationat shim locations which could lead to localized fatigue failure at shimlocations. The new method employs the bolt template 52 at the very topof the bolt assembly 60 with at least three sets of additional levelingbolts 53 and corresponding threaded bolt inserts 53 b suitable forembedment into concrete. The leveling bolts 53 and inserts 53 b may belocated outside or inside the bolt circle 60 a of the tower base, butdirectly under the tower base flange 301 a. This allows for continuityof the grout bed 90 a construction and provides an easy access forleveling bolts 53. Small cutouts 53 a connected at leveling boltlocations can be used. Another benefit of this leveling technique ishaving the ability to apply a continuous grout bed 90 a that is free ofconstruction joints, under tower base 301 in one session and to have theability to tension all the anchor bolts 56 in one session.

The present invention improves safety and accessibility aroundfoundations during construction and reduces hazardous conditions forconstruction crews. This goal is achieved by using reusable formsections 102 that are fitted with platform sections for forming anaccess platform around the foundation. The form may also connect to atleast one access ramp extending beyond the edge of the foundation. Theplatform and the ramp are fitted with a slip-resistant walking surfaceand the elevated ramps are provided with guardrails and designed toapplicable industry safety standards. Further, the relatively thin slabthickness minimizes the risk of worker injury during construction.

A transformer pad can be supported on precast concrete posts extendingvertically from the foundation.

Pedestal forms 102 may have openings fox running electrical andcommunication conduits there through thus preventing problems stemmingfrom randomly placing the conduits in areas that could compromise thestructural design.

The ribs 16 may have means for receiving and supporting prefabricatedtrays (or electrical duct banks) for housing power and communicationcables.

The foundation design can also be adapted for offshore wind turbineprojects. In this case the foundation 100 may be assembled on a barge ordry dock then transported or floated to its destination and lowered intoa prepared seabed location. The foundation can be weighed down in placeby backfilling it with suitable material. The offshore foundation 100may be configured to receive any type of offshore piers 404, suctionpiers 403, piles 400, micro-piles 401, anchors 404 or any combination ofthe above.

In another embodiment of the invention as shown in FIG. 42, an offshoreconcrete foundation 100 with high stiffness and improved fatigueresistant comprising: 1. a support slab-on-grade 20 of non-segmentedcontinuous construction covering the entire footprint of the foundationand having (horizontal) diametric and perimeter post-tensioningelements, 2. a central pedestal 10 integral with the supportslab-on-grade 20 of solid non-segmented construction and having verticalpost-tensioning elements and also having reinforcing elements of rebarto carry loads diametrically across the pedestal 10; 3. a cylindrical orconical stem 11 extending vertically above the pedestal 10 and beingfixed to the pedestal 10, and having a hollow cross section, of equalsize or smaller than that of the pedestal 10, and may be constructedwith segmented or non-segmented construction methods and could be madewith typical cast in place over the pedestal 10 by using typicalconstruction methods for tall cylindrical concrete structures such ascontinuous forming, successive pours, segmental construction withprecast concrete panels or other known construction methods usedconventionally for conical or cylindrical concrete structures such aschimneys, and the stem 11 is kept under heavy concentric verticalpost-compression stress by an array of circumferentially arrangedvertical post-tensioning elements 70, and the stem 11 may have an icecone 11 b, or tower receiving adaptor, integral with the top of stem 11,and the stem 11 having means for fixing a tower base 301 of a wind tower300, the stem 11 and the ice cone 11 b are vertically andcircumferentially prestressed with vertical and circumferential posttensioning elements, 4. ribs 16 integral with the support slab 20 andthe central pedestal 10, on top of the slab-on-grade, with pairs of ribs16 radially extending from opposite sides of the pedestal 10 withpost-tensioning elements extending radially and diagonally from thedistal ends 16 x of the ribs 16 through the pedestal 10 and keeping theribs 16 and the pedestal 10 under heavy eccentric post compressionstress and reinforcing dowels 46 extending from the ribs 16 into thepedestal 10 and spliced with pedestal 10 reinforcing, 5. deep perimeterbeams 190 extending continuously around the foundation, made of concreteintegral with the support slab-on-grade 20 and the ribs 16 and havingcontinuous perimeter or circumferential post tensioning elements. Whenthe concrete sets, the post-tensioning elements are jacked and theanchor bolts are post-tensioned such that the foundation is kept underheavy multi-axial post-compression.

The offshore foundation 100 is constructed on a barge or in a dry dockand then floated or transported to an offshore installation site andlowered to be placed over a prepared sea bed. A suitable backfillmaterial 13 is placed over the foundation 100 to stabilize thefoundation against overturning. Scour protection measures 13 b areprovided around the foundation. The foundation may be built with marinecement and marine grout and kept under heavy multi-axial horizontal andvertical prestress using bonded and grouted post tensioning systemsrated for double corrosion protection and suitable for a marineenvironment.

In another embodiment as shown in FIG. 40, an offshore foundation forwind turbines comprises the following elements: 1. A verticallyextending pedestal that is cast in situ, on a barge, out of concrete,the pedestal has an integral long stem 11 for receiving and supporting atower structure; 2. A substantially horizontal support slab 20 that iscast in situ, on a barge, out of concrete, the support slab 20 coveringan area of ground larger than that covered by the pedestal 10; 3. Aplurality of radial ribs 16 extending radially outwardly from thepedestal 10 and spaced around the pedestal 10, each rib beingprefabricated and being joined along the base thereof to the supportslab 20 when the support slab 20 is cast in situ and being joined alongan inner side thereof to the pedestal 10 when the pedestal 10 is cast insitu; 4. A plurality of prefabricated perimeter beams 190 spanningcontinuously, near the perimeter of the foundation 100, between ribs 16and supporting the slab 20; 5. Backfill 13 for weighing down thefoundation, resisting tower loads and providing scour protection 13 b.

When the concrete sets, the precast components will become integral witha cast-in-place components. Radial post-tensioning tendons extend fromthe distal end of one rib through the rib and the pedestal to the distalend of the opposite rib. Vertical post-tensioning is arranged in thepedestal 10 as well. The stem 11 and the ice cone 11 b may also benefitfrom circumferential post-tensioning 59 t.

The pedestal 10 has means for receiving and supporting a tower 300 orpylon. The upper portion of the pedestal 10 (the stem 11) may be made inmultiple consecutive cast in situ pours, depending on its height.Alternatively, the stem 11 may be made by joining precast segments withcircumferential 59 t and vertical 70 post-tensioning to form the stem 11as in segmented concrete tower construction.

In another embodiment of the invention, as shown in FIG. 42, a windturbine foundation may be fabricated on a barge with precast concreteelements. The barge surface is prepared with a non-bonding agent or athin membrane at the foot print where the foundation is to be built.Lower slab reinforcing mesh sections are assembled and placed on thebarge and the pedestal cage reinforcing is assembled at the center ofthe foundation. Upper slab reinforcing mesh sections 24 may follow afterthe slab post tension duct 58 dc is placed. Precast concrete ribs 16 areplaced in a radial array around the pedestal cage 50 and precastconcrete perimeter beams 190 are arranged around the perimeter of thefoundation 100 p. Post tensioning ducts 58 dc in the pedestal space 10and at perimeter beam-to-rib connections 59 dc are placed to pair withtheir corresponding duct in the precast members. Forms for the pedestal10 and for closure pours at rib-to-perimeter beam connections areinstalled. The slab concrete is poured followed by pedestal 10 concreteand closure pours at the rib-to-pedestal connections. The stem 11 isfabricated possibly in multiple consecutive pours depending on pedestalheight. The stem 11 design may incorporate an ice cone 11 b at its top.The post tensioning tendons are then installed. The jacking and groutingof tendons is then carried out. Some pylon sections may be installedprior to transportation. The finished foundation 100 is transported toits offshore installation site using a suitable means of transportationsuch as towing the barge.

In another embodiment of the offshore foundation comprises the followingelements as shown in FIG. 46: 1. A vertically extending pedestal 10 iscast in situ, on a barge or dry dock, out of concrete; 2. Asubstantially horizontal support slab 20 is cast in situ, on a barge ordry dock, out of concrete, the support slab 20 covering an area largerthan that covered by the pedestal 10; 3. A plurality of radial ribs 16extends radially outwardly from the pedestal 10 and spaced around thepedestal 10, each rib being prefabricated and being joined along thebase thereof to the support slab when the support slab 20 is cast insitu and being joined along an inner side thereof to the pedestal 10when the pedestal is cast in situ, each rib has an integral pier 180 forreceiving a leg 210 of lattice tower 200; 4. A plurality of perimeterbeams 190 spanning continuously, near the perimeter of the foundation100, between ribs 16 and supporting the slab 20, optionally eachperimeter beam can be prefabricated; 5. A lattice tower 200 has aplurality of legs 210 structurally connected to the integral piers 180in the ribs 16, the lattice tower 200 has, at its top, a means forreceiving and structurally supporting a pylon or a tower 300; 6.Suitable offshore backfill 13 for weighing down the foundation,resisting tower loads and providing scour protection 13 b.

When the concrete sets, the pre-cast components will become integralwith the cast-in-place components. Radial post-tensioning tendons extendfrom rib ends 16 x to the opposite rib ends 16 x across the pedestal 10.Vertical post-tensioning is arranged in the pedestal 10 as well. Thestructural behavior is improved by the added compression in all ribs 16,edge beams 190, slab 20 and center pedestal 10.

The lattice tower 200, preferably incorporating 3-dimensional trusses200 tr, transfers the pylon loads down to the concrete foundation 100.The lattice tower 200 may get connected to the concrete foundation priorto transportation or it can be connected to the foundation at finaloffshore installation site.

In another embodiment of the invention as shown in FIG. 46, a windturbine foundation is fabricated on a barge with precast concreteelement as following. The barge surface is coated with a non-bondingagent or covered with a thin membrane at the foot print where thefoundation 100 is to be built. Lower slab reinforcing mesh sections areassembled and placed in the slab area and the pedestal cage 50reinforcing is assembled at the center of the foundation. Upper slabreinforcing mesh sections may follow after the slab post tension ductsare placed. Precast concrete ribs 16 are placed in a radial array aroundthe pedestal cage 50 and precast concrete perimeter beams 190 arearranged around the perimeter of the foundation 100 p, post tensioningducts in the pedestal space and at perimeter beam-to-rib connections areplaced to pair with corresponding duct in the precast members. Forms forthe pedestal and for closure pours at the rib-to-perimeter beamconnections are installed. Slab concrete is poured followed by pedestalconcrete and closure pours at rib-to-pedestal connections. A latticetower 200 structure is prefabricated and mounted atop the concretefoundation 100. The foundation is transported to the installation siteusing a suitable means of transportation. The seabed is prepared forreceiving the foundation by placing a sub-base of suitable material suchas crushed stone. The foundation is backfilled and scour protectionmeasures 13 b are installed.

In another embodiment of the invention, as shown in FIG. 31, the stem 11is prefabricated separately and provided with a means for connecting tothe pedestal 10, preferably an array of vertical post tensioning dowels70 extended through the pedestal 10 and the stem 11 or other segmentalpost tensioning joining methods may be used. The pedestal may be fittedwith a means for receiving the prefabricated stem 11 based on segmentalpost tensioning and grouting construction methods.

Piles, Micro-piles or piers or suction piers or anchors can be used withthe offshore foundation 100 in a similar manner as previously describedin the application. In this case vertical sleeves will be arranged inthe foundation to receive an array of piles or anchors extending throughthe foundation, to allow for additional loading capacity and improve thestability or the foundation. Piles are secured to the foundation byfilling the sleeves with marine grout.

Under some conditions, the use of piles, piers or suction piers oranchors may eliminate the slab 20 and/or the perimeter beams 190 fromthe design.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that, within the scope of the invention.

The invention claimed is:
 1. A wind turbine foundation for a pylon of awind tower comprising: an excavation pit at a foundation constructionsite; a continuous support slab on grade that is cast in situ out ofconcrete over a prepared subgrade at the bottom of the excavation pitbelow grade elevation and having generally horizontal rebar meshtherein; a vertically extending and laterally braced tower receivingpedestal, the pedestal being integral with the support slab andencompassing vertical reinforcing cage and vertical post tensioningelements therein; a plurality of concrete ribs on top of and integralwith the support slab and integral with the pedestal, the pedestal andthe ribs being conjointly cast, in situ, out of concrete, the ribshaving connection elements extending therefrom into the pedestal andinto the slab such that the ribs, slab and pedestal form a monolithicfoundation, the ribs being tapered downwardly and outwardly from thepedestal and with inner ends of the ribs having a height adjacent to theheight of the pedestal to thereby provide lateral bracing andconfinement to the pedestal; and compacted backfilling material placedover and around the foundation to substantially bury the foundation andrestore grade such that the pedestal protrudes above grade and isadapted to support a base of a wind tower near grade elevation.
 2. Awind turbine foundation as in claim 1, wherein the pedestal is of solidcore construction that is kept, through most of its height, under acombination of lateral structural support and confinement afforded bythe ribs and high vertical post-compression stress.
 3. A wind turbinefoundation as in claim 1, wherein the vertical post tensioning elementscomprise a vertically extending bolt cage assembly laterally confinedbetween the ribs and comprising an embedment element proximate a base ofthe pedestal and a tower base at a top of the pedestal with bondprotected vertical anchor bolts extending between the embedment elementand the wind tower base for vertical post tensioning of the foundation,as tension force is applied to the vertical anchor bolts by threadednuts tightened on the anchor bolts at a top of an activated one of thevertical post tensioning elements to provide post tensioning forcesufficient to ensure no loss of contact between the tower base and thefoundation under operating loads of a wind turbine.
 4. A wind turbinefoundation as in claim 3, wherein a bearing element is placed below thetower base at the top of the pedestal.
 5. A wind turbine foundation asin claim 4, wherein the foundation further comprises a tower base flangedisposed at the bottom of the wind tower when placed atop the pedestaland a grout bed between the tower base flange and the pedestal, whereinthe vertical post tensioning elements protrude above the pedestal andthrough the grout bed to connect the wind tower base to the pedestalwhile keeping the grout bed and the junction between the tower base, thepedestal and the ribs under post compression stress, reduced stressamplitudes and substantial lateral confinement.
 6. A wind turbinefoundation as in claim 4, wherein the foundation further comprises abearing washer plate placed between the tower base and the top of thepedestal.
 7. A wind turbine foundation as in claim 1, wherein thefoundation has a thickened slab perimeter to stiffen the foundation. 8.A wind turbine foundation as in claim 1, wherein the foundation isformed with its total width being more than twice its total height suchthat structural components have a predetermined ratio of concrete massto surface area to limit a risk of thermal cracking due to heat ofhydration.
 9. A wind turbine foundation as in claim 1, wherein thefoundation further includes one or more post tensioning elements inpreselected locations including the ribs, slab and a thickened slabperimeter.
 10. A wind turbine foundation as in claim 9, wherein the posttensioning elements form a three-dimensional prestressing network in thefoundation that keeps the structural components under multi-axial postcompression stress and lateral confinement to lower stress rangeamplitudes and yielding higher stiffness, improved crack control, lowerdeflections and improved fatigue resistance.
 11. A wind turbinefoundation for a tower structure as described in claim 1, furthercomprising a ground support system selected from a group including apiling system, a pier system and an anchor system, connected to thefoundation and extending into the ground below the foundation.
 12. Awind turbine foundation as in claim 1, wherein the concrete support slabhas a plurality of layers of rebar grid reinforcing contained therein.13. A wind turbine foundation as in claim 1, wherein the slab workscompositely with the ribs and includes a plurality of reinforcingelements extending along the junctions of the slab with the ribs andextending towards the pedestal.
 14. A wind turbine foundation as inclaim 1, wherein the concrete support slab has one of a flat and slopedtop surface with an unrestricted perimeter configured to facilitatedrainage whereby ground water is not retained over the slab.
 15. A windturbine foundation as in claim 1, wherein the concrete support slab hasan accessible top surface with an unrestricted perimeter to facilitatecompaction of backfill material using roller compactors working over thefoundation.
 16. A wind turbine foundation as in claim 1, wherein thepedestal is of solid core construction and the ribs extend radially inpairs on diametrically opposite sides of the pedestal such that towerloads are resisted by pairs of ribs, wherein each pair of ribs form acontinuous, non-segmented, laterally supported, reinforced concretegirder extending between distal ends of the foundation with continuouscomposite action from the slab-on-grade.
 17. A wind turbine foundationas in claim 1, wherein the pedestal comprises a bolt cage assembly thatis confined within a vertically extending outer reinforcing cageassembly made of an array of vertical bars with hooked ends arrangedbetween the ribs, the bolt cage assembly being also confined by an arrayof rib vertical stirrups arranged at intervals along the length of eachrib with a concentration of stirrups in proximity to the bolt cage. 18.A wind turbine foundation as in claim 17, wherein pedestal furthercomprises an inner reinforcing cage located such that the bolt cage isconfined between the outer and inner pedestal reinforcing cages.
 19. Awind turbine foundation as described in claim 1 further comprising aconcentration of primary foundation reinforcing elements comprising anarray of upper rib reinforcing elements extending diagonally along asloped top of the ribs and continuing into the pedestal between thevertical post tensioning element; and an array of horizontal reinforcingelements extending along a junction of the rib and the slab andcontinuing towards a middle of the foundation; and an array of verticalstirrups of variable height arranged at intervals along the length ofeach rib with a concentration of stirrups in proximity to the pedestal.20. A wind turbine foundation as in claim 1, wherein the foundationfurther comprises a lower horizontal bottom reinforcing mesh proximatethe bottom of the pedestal; an upper horizontal top reinforcing meshnear the top of the pedestal; and an array of vertical reinforcing barsextending therebetween, wherein the upper and bottom reinforcing meshesoverlap with upper and lower horizontal reinforcing dowels to carry theloads across the pedestal.
 21. A wind turbine foundation for a towerstructure as described in claim 1, further comprising additionalfoundation reinforcing elements wherein the additional foundationreinforcing elements are selected from specific reinforcing groupsincluding: a) top slab reinforcing mesh, b) bottom slab reinforcingmesh, c) slab shear reinforcing extending between top and bottom slabreinforcing meshes, d) flexure reinforcing concentrated at the bottomand the top of the ribs, e) vertical stirrups in the ribs for shearreinforcing and tightly spaced in high shear zones at rib inner end, f)rib skin reinforcing on each face of the rib, g) rib diagonalreinforcing for diaphragm action at inner zones of the ribs, h) andbursting and splitting reinforcing made of horizontal hairpins extendingbetween the rib skin reinforcing, i) straight, hooked and U-shapedhorizontal dowels for embedment into the pedestal and j) verticaldowels, at the bottom of the ribs for composite action with the slab andcombinations thereof.
 22. A wind turbine foundation as in claim 21,wherein the reinforcing elements are partially preassembled in cages andplaced within forms in the excavation pit.
 23. A wind turbine foundationas in claim 1, wherein the slab is cast in situ out of concrete over theprepared subgrade, in a single concrete pour and is free of constructionjoints within.
 24. A wind turbine foundation as in claim 1, wherein theribs and the pedestal are conjointly cast in situ out of concrete overthe slab and their junctions are free of construction joints and havingshear and flexure reinforcing elements within.
 25. A wind turbinefoundation as in claim 1, comprising strength-accelerator admixtureadded to the concrete mix for the slab to facilitate the construction ofthe foundation in one continuous concrete pour.
 26. A wind turbinefoundation as in claim 1, comprising a protective coating applied to thefoundation surface to increase its chemical resistance.
 27. A windturbine foundation as in claim 1, wherein the construction is such thatnatural frequencies of the constructed foundation do not interfere withoperating frequencies of a wind turbine generator installed atop thetower on the foundation.
 28. A wind turbine foundation as in claim 1,wherein the mechanical properties of the backfilling material enable theburied foundation to resist wind turbine loading by interlocking withthe buried foundation surface areas and exerting substantial verticaland lateral pressure from material weight, active and passive lateralpressure and friction acting on the buried surfaces of the foundation asit resists turbine loads.
 29. A wind turbine foundation as in claim 1,wherein the foundation includes conduits and means for receiving andsupporting electrical cables, duct banks and communication cables atpredetermined locations.
 30. A foundation as in claim 25 wherein thefoundation includes a reinforcing element comprising: (a) an array ofdiagonally extending upper flexure reinforcing members running in theribs with deep dowels extending into the pedestal through prestressedconcrete therein; (b) an array of lower flexure reinforcing membersrunning in the slab along the length of the ribs with deep dowelsextending into the pedestal; and (c) a plurality of vertical stirrupsembedded in the slab and extending into the ribs along the length of theribs and the said array of stirrups operatively couple the upper andlower foundation flexure reinforcement members.
 31. A method forconstructing a foundation having a continuous slab, a pedestal, and aplurality of tapered ribs of cantilevered girder construction comprisingthe following steps: a) preparation of the site including the placementof a construction barrier over a flattened subgrade at the bottom of anexcavation pit; b) placing an anchor-bolt cage and foundationreinforcement at the excavation pit; c) installation of formworkincluding a slab form, an array of cantilevered rib forms and a pedestalform; d) placement of flowable concrete inside the said forms andallowing the concrete to harden such that the ribs become integral withthe support slab and integral with the pedestal when the concretehardens forming a monolithic foundation; (e) formwork removal; (f)concrete curing, wherein step (b) is carried out in the followingsub-steps: (b1) placing the reinforcement of the slab on grade with anupper and a lower horizontal rebar meshes therein; (b2) placing inposition the anchor-bolt cage with leveling and positioning meansconnected to the anchor-bolt cage to ensure its proper positioning andvertical alignment; (b3) placing reinforcement of the pedestalcomprising a vertically extending and tightly meshed pedestal cageassembly around the said anchor-bolt cage; and (b4) placing an array ofrib reinforcement cages around the said anchor-bolt cage, the said ribreinforcing cages being of tapering height and having vertical stirrupsand diagonally extending upper flexure reinforcing with dowels extendinginto the pedestal cage and vertical dowels extending from the ribs intothe slab at spaced intervals; and (g) step (f) is carried out by onsitecuring of the foundation elements in a controlled manner.
 32. A methodfor constructing a wind turbine foundation as in claim 31, furthercomprising placing compacted backfilling material over and around thefoundation to substantially bury the foundation and restore grade suchthat the pedestal protrudes above grade with anchor bolts protrudingthrough the pedestal to support a base of a wind tower near gradeelevation with the ribs substantially buried with inner ends thereofhaving a height at least a predetermined fraction of that of thepedestal.
 33. A method for constructing a wind turbine foundation as inclaim 32, further comprising placing a tower base section with a baseflange having bolt apertures for receiving the anchor bolts of theanchor bolt cage on top of the pedestal.
 34. A method for constructing awind turbine foundation as in claim 33, further comprising providingmeans on the top of pedestal for having a gap between the pedestal andthe tower base flange and placing grout therein.
 35. A method forconstructing a wind turbine foundation as in claim 34, furthercomprising placing nuts on the anchor bolts and tensioning the boltsafter the grout is hardened.
 36. A method for constructing a windturbine foundation as in claim 35, further comprising placing a windtower section on top of the tower base section and installing windturbine rotor-nacelle assembly on top of the wind tower sections.
 37. Amethod for constructing a wind turbine foundation as in claim 31,further comprising pouring the slab on grade in a first pour and pouringthe ribs and the pedestal conjointly in a second pour.
 38. A method forconstructing a wind turbine foundation as in claim 31, furthercomprising pouring the foundation in one continuous pour.
 39. A cast insitu foundation comprising a continuous concrete support slab havinghorizontal rebar therein, a vertically prestressed concrete pedestalintegral with the support slab having vertical rebar therein, aplurality of concrete ribs on top of and integral with the support slaband integral with the pedestal, the ribs being horizontal cantileveredgirders fixed at their inner ends to the pedestal and taperingdownwardly as they extend outwardly from the pedestal, the ribs havingdeep lateral connection elements extending from the ribs into thepedestal through prestressed concrete and further connection elementsextending from the ribs into the slab.
 40. A foundation as in claim 39wherein the foundation includes an array of vertical post tensioningelement in the pedestal.
 41. A cast in situ foundation as in claim 39wherein the foundation has a thickened slab perimeter to stiffen thefoundation.
 42. A cast in situ foundation as in claim 39 wherein thefoundation further includes one or more post tensioning elements inpreselected locations including the ribs, slab and a thickened slabperimeter.
 43. A foundation comprising: (a) a concrete support slabhaving horizontally extending rebar therein; (b) a concrete pedestalintegral with the support slab, the pedestal having vertically extendingrebar therein; (c) a plurality of concrete ribs of a cantilever girderconstruction on top of and integral with the support slab and with thepedestal; (d) connection elements extending from the plurality of ribsinto the pedestal and into the slab, wherein the ribs are tapereddownwardly and outwardly from the pedestal; and (e) a plurality ofvertically oriented stirrups of tapering height embedded in the slab andextending toward, but short of, a top of the plurality of ribs along alength of the ribs and with a first set of rebar running between thestirrups so as to provide both diagonally extending upper flexurereinforcing along the top of the ribs and horizontally extending lowerflexure reinforcing in the slab along the length of the ribs.